The front-runners in each party – Republican Donald Trump and Democrat Hillary Clinton – are hoping that Super Tuesday will help them distance themselves even further from their rivals, as challengers Marco Rubio, Ted Cruz and Bernie Sanders are looking to gain ground in Tuesday's delegate-rich dash across the country.
Voters from Vermont to Colorado, Alaska to American Samoa and a host of states in between were heading to polling places and caucus sites on the busiest day of the 2016 primaries.
The contests come at a turbulent moment for Republicans as they grapple with the prospect of Trump becoming the party's nominee. Rubio and Cruz are engaged in a frantic effort to stop the billionaire, but it was unclear whether they'd made their move too late.
"This is a movement," Trump told supporters at a rally Monday evening in Georgia. "I'm just a messenger, folks."
Democrats will vote in 11 states and American Samoa on Tuesday, with 865 delegates up for grabs. Republicans will vote in 11 states, with 595 delegates at stake.
Trump was seeking to sweep the South, which would be a massive blow for Cruz. The Texas senator, a favorite of the region's social conservatives and evangelical Christians, expected the South to be his firewall, but now is simply hoping to emerge with a victory in his home state.
Rubio's goal on Super Tuesday is even more modest. He's seeking to stay competitive in the delegate count and hopes to pull off a win in his home state of Florida on March 15.
The Florida senator has cast himself as Republicans' best chance to win in a general election and has received a flood of endorsements from GOP officials after other more mainstream candidates dropped out. But he's failed to win a state so far, raising questions about his strategy for topping Trump.
Like Trump, Clinton has won three of the four early voting contests, including a thrashing of rival Sanders in South Carolina on Saturday. Her victory there was due to overwhelming support from black voters, putting her in position for a strong showing in several Southern states with large African-American electorates that vote Tuesday.
Clinton has increasingly turned her attention to Trump in recent days, casting herself as a civil alternative to the insults and bullying that have consumed the Republican race.
"What we can't let happen is the scapegoating, the blaming, the finger pointing that is going on the Republican side," she told voters gathered in Springfield, Massachusetts. "It really undermines our fabric as a nation. So, I want to do everything I can in this campaign to set us on a different course."
Sanders, who has energized young voters with his call for a political revolution, was seeking to stay close to Clinton in the South and pick up victories in states including Minnesota and his home state of Vermont. But Sanders faces tough questions about whether he can rally minorities that are core Democratic voters.
Republicans spent months largely letting Trump go unchallenged, wrongly assuming that his populist appeal with voters would fizzle. Now party leaders are divided between those who pledge to fall in line behind Trump if he wins their party's nomination and others who insist they can never back him.
An Associated Press survey of GOP senators and governors across the country showed just under half of respondents would not commit to backing Trump if he's the nominee. Their reluctance foreshadowed a potentially extraordinary split in the party this fall.
"If he becomes the nominee the Democrats are going to savage him, no question about it," GOP Utah Sen. Orrin Hatch said. His Republican colleague from Arizona, Jeff Flake, said he was "still holding out hope" that he wouldn't have to make the choice about supporting Trump.
The worries among Republicans appeared to grow after Trump briefly refused to disavow former Ku Klux Klan leader David Duke during a television interview. Trump said he had not understood the interviewer who first raised the question about Duke, and he did later repudiate him.
"Clearly that's caused a lot of concern," said Texas Sen. John Cornyn, one of the lawmakers who would back Trump as the nominee. "We've got about two weeks to find out whether he's going to continue his juggernaut or whether somebody else in going to pull into the pole position. But right now it looks like it's his to lose."
Based on reporting by The Associated Press.