President Donald Trump on Monday will ask Congress for an additional $8.6 billion to pay for a wall along the southern border with Mexico, a senior administration official confirmed to NBC News.

The demand, part of the administration's budget request for fiscal year 2020, is more than six times what Congress allocated for border projects in each of the past two fiscal years and slightly more than Trump has corralled by invoking emergency powers this year.

The request faces a steep uphill battle as Democrats, who oppose the wall as unnecessary and immoral, control the House, making the request's passage unlikely.

Reuters first reported the administration's planned request, citing officials familiar with the matter.

In a joint Sunday statement, Senate Minority Leader Chuck Schumer, D-N.Y., and House Speaker Nancy Pelosi, D-Calif., vowed that the president's proposal would be defeated just as his previous funding request was shot down earlier this year following the lengthiest government shutdown in U.S. history.

"President Trump hurt millions of Americans and caused widespread chaos when he recklessly shut down the government to try to get his expensive and ineffective wall, which he promised would be paid for by Mexico," the two Democrats said in a statement. "Congress refused to fund his wall and he was forced to admit defeat and reopen the government. The same thing will repeat itself if he tries this again. We hope he learned his lesson."

"At a time when our country faces challenges about jobs for the future, this money would better be spent on rebuilding America, and on education and workforce development for jobs for the 21st Century," they continued.

Speaking on "Fox News Sunday," Larry Kudlow, the White House's chief economic adviser, acknowledged the request and said the "whole issue of the wall, of border security, is of paramount importance."

"We have a crisis down there," he said. "I think the president has made that case very effectively."

The proposal comes on the heels of a bruising battle with Congress over wall funding that resulted in a five-week partial federal government shutdown that ended in January, and could touch off a sequel just ahead of a trifecta of ominous fiscal deadlines looming this fall.Regardless of whether Congress passes it, the budget request could help Trump frame his argument on border security as the 2020 presidential race begins to take shape, with the president seeking re-election.

Funding legislation needs to be passed before Oct. 1 — the start of the 2020 federal fiscal year — or the government could shut down again. If Congress and the White House fail to agree to lift mandatory spending caps set in a 2011 law, steep automatic cuts in many programs would kick in. Around the same time, Trump and lawmakers must agree to lift the debt ceiling, or risk a default, which would have chaotic economic fallout.

Trump's wall request is based on a 2017 plan put forward by Customs and Border Protection officials to build or replace 722 miles of barrier along the border, which in total is estimated to cost about $18 billion.

For fiscal year 2019, Trump demanded $5.7 billion in wall funds, but Congress appropriated only $1.375 billion for border fencing projects.

Following the rejection of his wall funding demand, Trump declared the border was a national emergency — a move opposed by Democrats and some Republicans — and redirected $601 million in Treasury Department forfeiture funds, $2.5 billion in Defense Department drug interdiction money and $3.6 billion from a military construction budget, for total spending of $8.1 billion for the wall.

Many lawmakers accused Trump of overstepping his constitutional powers by declaring an emergency to free up the funds. The House has already voted to revoke the emergency, and the Senate is likely to do the same this week. Trump is expected to veto the resolution.