The U.S. was steeling itself for Hurricane Matthew as the monster storm pounded Haiti on Tuesday and set its sights on the Sunshine State and the Carolinas.

"We cannot rule out a direct hit on our state," Florida Gov. Rick Scott warned. "These storms can change direction on a dime."

Scott said that while the latest projections show the storm passing along the east coast of Florida starting on Wednesday, it could still do immense damage.

"Hurricane Matthew is a deadly storm and has already killed multiple people," he said. "Even if it stays off course, expect it to get tropical storm strength winds" and 4 to 7 inches of rain.

Scott spoke after the National Hurricane Center issued tropical storm and hurricane watches for Florida for a Category 4 storm packing 145 mph winds.

The tropical storm watch extends from the Seven Mile Bridge in the Florida Keys to Deerfield Beach in Palm Beach County and includes Lake Okeechobee. The hurricane watch extends from Deerfield Beach to the Brevard-Volusia county line.

"It hasn't gotten better for us, it's gotten worse," Scott said. "And things can become much worse than what's projected at this time."

Scott said 200 members of the National Guard have been activated and said "evacuation orders might be issued today" for some of the most vulnerable coastal counties. He urged Floridians to heed the order to get out if and when it's issued.

"During the middle of a storm, they can't show up to save," he said of the rescue workers. "Leave before it's too late."

Florida Sen. Marco Rubio echoed Scott's remarks.

"This is a dangerous and life-threatening storm, the likes of which our state has not seen in a decade," he said. "Florida is currently in Matthew's path, and Floridians need to take today to prepare."

Meanwhile, worried Florida residents picked the grocery store shelves clean of staples like bottled water and batteries.

"I got scared because all that was left at Publix was just the pricey water," Simone Corrado of Davie, Florida told the Associated Press. "They really put the fear into you here. On the television screen every few minutes is the 'beep, beep, beep' storm alert."

South Carolina Gov. Nikki Haley was expected to outline the precautions her state is taking later Tuesday.

Like his Florida counterpart, North Carolina Gov. Pat McCrory declared a state of emergency on Monday. And on Tuesday, authorities began evacuating Cape Lookout National Seashore and the barrier islands on the North Carolina coast.

Meanwhile, the Federal Emergency Management Agency (FEMA) has already deployed extra hands and supplies to Florida, Georgia, North and South Carolina while the White House postponed plans for President Obama to campaign for Hillary Clinton on Wednesday in Miami and Tampa, Florida.

Instead, Obama will head to FEMA headquarters on Wednesday to track the storm and get an update on the federal emergency response.

Matthew is the most powerful Atlantic storm since 2007. NBC News meteorologist Bill Karins said its path appears to be similar to that of Hurricane Floyd in 1999, which forced the evacuation of 2.6 million people across five states.

It's likely to weaken to a Category 2 or 3 storm by the time it reaches the Georgia and North Carolina coasts around Friday, Karins said.