The 2016 presidential election proved the maxim that every vote counts. Donald Trump squeaked out an upset victory by fewer than 80,000 votes spread across three swing states.

As the 2020 contest approaches, the laws that determine who makes it to the polls are in flux. Legislators in some states that may be crucial to an Electoral College victory have passed or proposed measures that would make it harder for residents to vote. Some states have done the opposite, removing barriers to voting. Still others have contended with challenges to existing laws — laws critics say were meant to restrict access to the polls or make victory elusive for certain candidates.

Here are nine states where the laws have been changed or challenged:

Florida

Last fall, citizens of this presidential battleground voted nearly two-to-one to give most felons the right to vote once they had completed prison, parole and probation.

The passage of Amendment 4 seemed like it might transform Florida's electorate, adding as many as 1.4 million voters to the rolls, a high percentage of them black. But then Florida's Republican-majority legislature stepped in.

In May, the state House and Senate passed a bill that added another step to the restoration of voting rights for felons. They would have to pay any and all outstanding court fees before becoming eligible to vote. Gov. Ron DeSantis, a Republican elected last November, signed the bill in late June. He said he considers court fines to be part of prison sentences, and thus the legislature was just implementing the amendment as written.

Florida, with 29 electoral votes, is one of the nation's most politically competitive states. Barack Obama won the state in 2008 and 2012 and Trump won in 2016. The 2018 governor's race was decided by less than 34,000 votes out of more than 8 million cast.

About a quarter to a third of the voters who are eligible to have their voting rights restored under Amendment 4 are African-American, a group that votes for Democratic candidates by a margin of more than nine to one.

Democrats across the nation, including Hillary Clinton, Sen. Cory Booker, D-N.J., and Rep. Alexandria Ocasio-Cortez, D-N.Y., have blasted the requirement that felons pay court fines before voting as "a poll tax."

Ohio

Like Florida, Ohio is a stepping stone to the White House. Trump won the state easily in 2016, Obama prevailed twice, and George W. Bush's narrow win in 2004 secured his reelection.

Since the 1990s, Ohio has had a "use it or lose it" voting policy. Election officials send notices to anyone who fails to cast a ballot during a two-year period. Those who do not respond and don't vote over two more federal election cycles are dropped from the list of registered voters. Between 2011 and 2014, nearly 2 million names were purged. Ohio currently has about 8 million registered voters.

Supporters say it helps "clean the deadwood off the rolls" and prevent fraud. Opponents say it disproportionately affects poor and minority voters.

Ohio has had GOP secretaries of state for all but four years out of the past 28. Republican secretaries introduced and have maintained the "use it or lose it" policy.

In 2016, a voter who had been purged from the rolls joined with the non-profit Demos to sue the state over "use it or lose it." In June 2018, the Supreme Court ruled in favor of Ohio's voter purge system by 5-4.

After the Supreme Court victory, Ohio Secretary of State Jon Husted called the decision "a victory for election integrity, and a defeat for those who want to use the federal court system to make election law across the country."

Husted suggested that Ohio's law could serve as a model for other states. Dan Tokaji, an Ohio State professor who worked on the challenge to Ohio's law, agreed that other states would take notice.

Said Tokaji, "I would expect that we will probably see more states — at least ones in which Republicans are in control — trying to adopt a use it or lose it approach."

Arizona

In January, Republicans in the Arizona legislature tried to pass a bill that would have applied the "use it or lose it" rules to the states Permanent Early Voting List (PEVL). About 80 percent of state voters are signed up for the list, which allows voting by mail, and because of the PEVL about three-quarters of state ballots are cast by mail.

The bill would have purged voters who missed two election cycles from the list that permits early voting, which includes about 70 percent of Arizona voters, according to Arizona Advocacy Network Executive Director Joel Edman.

The bill passed the Senate but failed to receive a vote from the full House.

"Being on the PEVL has proven to be a strong driver of turnout among those least likely to vote," said Joel Edman, director of executive director of Arizona Advocacy Network, a voting rights advocacy group. "Removing those marginal voters from the PEVL would only further depress their turnout rates."

Tennessee

A bill signed into law in May will make it harder for activists to register new voters in bright red Tennessee. The law is expected to have its greatest impact on low income, Hispanic and African-American voters.

Passed by the state's heavily GOP legislature and signed by GOP Gov. Bill Lee, the law requires anyone organizing voter registration drives to undergo training administered by the coordinator of elections. Violators of the new law are subject to Class A misdemeanor charges.

Following the bill's passage, the American Civil Liberties Union filed a federal lawsuit challenging the "strict, unnecessary, and irrational restrictions on community-based voter registration speech and activity."

Proponents of the law said it was meant to prevent fraudulent voter registration.

In its suit, the ACLU said, "this justification merely serves as pretext for a different, more insidious purpose: to inhibit successful voter registration efforts by community-based organizations."

During the 2018 election cycle, civic organizations across Tennessee "made a concerted push" to register thousands of eligible citizens, according to the suit.

"Much of this community-based voter registration activity took place among communities of color and other underserved populations," the lawsuit says.

Prior to the bill's signing, Republican Secretary of State Tre Hargett said that improperly filled out and incomplete registration forms were a burden on local election offices.

"This bill will help ensure all who want to vote have the ability to do so and will enhance the security and integrity of elections," he said.

A spokesperson for Hargett told NBC News that the required voter registration training "will ideally be 30 minutes but no more than an hour."

New York

New York Democratic lawmakers sponsored a series of reforms intended to make voting easier in the bright blue state, where turnout consistently lags behind the national average.

State lawmakers passed a bill that — starting in 2020 — allows New York voters 10 days of early access to the ballot box prior to any special, primary, or general elections.

Lawmakers also approved a bill that allows 16 and 17-year-olds to pre-register to vote, a bill that automatically transfers voter registrations for people who move within the state and a bill consolidating federal and state primaries.

In 2017, New York Gov. Andrew Cuomo signed an executive order restoring the right to vote for parolees.

Myrna Perez, director of the Brennan Center for Justice's Voting Rights and Elections Program, said the moves were overdue.

"For a very long time, New Yorkers didn't have the same sort of modernity when it came to how their elections were run," she said.

Perez said that although voter "turnout is not the only measure of a healthy democracy," she is "hopeful that this increase in access and this increase in responsiveness" will increase turnout.

There are currently bills in New York's Democratic-controlled state senate that would expand voting even more by allowing same-day voter registration and no-excuse absentee voting.

California

The California Voter's Choice Act allows counties to conduct elections under a model that "provides greater flexibility and convenience for voters," according to California's Secretary of State. The law was passed and signed in 2016, but didn't take effect until 2018.

Counties that opt into the program mail every registered voter a ballot, allow voters to cast a ballot at any vote center within their county and expand in-person early voting.

Five of California's 58 counties adopted the new system in time for the 2018 general election, and those counties witnessed about a three percent increase in voter turnout, according to an independent study. Turnout among Latinos, Asians and young voters rose "sharply" from 2014 to 2018 in counties that adopted the new system compared to those that maintained the standard system.

Matt Barreto, a pollster and professor of political science at UCLA, said turnout was up across the whole state in 2018, "independent of the voting system in place," but that the new law had further increased turnout.

Seven different House districts flipped from Republican to Democrats in 2018, but none of those districts were in counties that adopted the new system. Fresno County, however, which had one of the closest fought House races, will be adopting the new system for 2020.

Fourteen counties have adopted the system for 2020.

Georgia

Just weeks before the November 2018 gubernatorial election, a federal judge ordered Georgia election officials to stop rejecting absentee ballots and applications because of mismatched signatures without giving voters the opportunity to fix the issues.

The October 2018 order came after more than 5,000 absentee ballots had been rejected. Georgia's second most populous county, Gwinnett, which is also more than half black, Asian and Latino — had rejected 465 ballots. Ultimately, about 7,000 ballots statewide were rejected, according to an investigation by the Atlanta Journal-Constitution.

Republican Brian Kemp ultimately won the governor's race by fewer than 55,000 ballots out of nearly 4 million cast. Incumbent Republican Rep. Rob Woodall won reelection to the U.S. House from the seventh district, which includes Gwinnett County, by 433 votes.

Mississippi

Four plaintiffs filed a federal lawsuit in May, hoping to change Mississippi’s voting system in time for the November governor's race. (Like a handful of other states, Mississippi has off-year elections.)

In Mississippi, candidates for statewide office must win not just a majority of the popular vote, but at least 62 of the 122 state House of Representatives districts. The system was adopted as part of Mississippi's 1890 constitution.

The lawsuit notes that the drafters of the constitution spoke openly, 129 years ago, about their desire to prevent black control of state government.

"The architects of this system for electing candidates to statewide office," states the lawsuit, "had one goal in mind: entrench white control of State government by ensuring that the newly enfranchised African-American citizens — who, at the time, constituted a majority of the State's population and some of whom had been elected to statewide offices — would never have an equal opportunity to translate their numerical strength into political power."

Mississippi's current population is 37 percent African-American, the highest share in the nation. Voting in the state is racially polarized, with whites voting overwhelmingly Republican and blacks solidly Democratic. Because of the polarization and because of the concentration of whites and blacks in specific state house districts, asserts the lawsuit, a black candidate would probably have to win 55 percent of the popular vote to also win the number of districts required under the Reconstruction-era law.

A federal judge has set deadlines of July 15 for the state to respond to the plaintiffs's arguments and July 20 to respond to plaintiffs’ request for an injunction that would keep the requirement from being in effect during this fall's election.

Former U.S. Attorney General Eric Holder is representing the plaintiffs.

Wisconsin

In November 2018, Democrats swept Republicans out of Wisconsin's top three statewide offices. Turnout was the highest in two decades, but the margins were thin — GOP Gov. Scott Walker lost reelection by fewer than 30,000 votes out of 2.7 million cast, while the incumbent attorney general lost by 18,000 votes.

A close analysis showed Democrats owed their victories to increased margins in the blue bastions of Madison and Milwaukee, including 13 wards in Madison that are home to a high concentration of University of Wisconsin students.

Before Walker left office, the lame duck GOP legislature convened a special session. Among the laws that the legislature passed, and Walker signed, were two that would have likely cut turnout in both Madison and Milwaukee in the next election. As of July, neither will take effect.

One law prohibited voters from using expired student IDs as identification when they show up to vote. The other limited early voting to two weeks before an election. Dane County, which includes Madison, and Milwaukee County both allow early voting up to six weeks before an election.

Republicans said the two-week limit on early voting would create a level playing field, as some communities had previously permitted more time to vote early than others.

"I like early voting," said Walker, explaining his support for the law. "I just like it to be fair."

The courts struck down most of the laws passed in December's special session, including the limits on early voting. The Wisconsin Supreme Court then reinstated the laws — but not the limit on early voting. For now, pending further legal challenge, the limits on early voting will not become law.

The lame-duck law barring expired student IDs will also not take effect. The director of the state Elections Commission said that barring any additional legal decisions, the rules for the use of identification in future elections will remain the same as they were for the 2018 general election.