Out front and looking ahead, Donald Trump and Democrat Hillary Clinton hope to begin charting a final path toward the general election on Super Tuesday, a delegate-rich day of primary contests likely to reveal candidates' strengths — and weaknesses — with a broad swath of American voters.
Elections were being held in a dozen states, from Vermont to Colorado, Alaska to American Samoa, and a host of locations in between.
WATCH LIVE COVERAGE FROM ABC NEWS IN THE VIDEO PLAYER BELOW
Trump and Clinton entered Super Tuesday having each won three of four early voting contests. Strong showings for both on Tuesday could start putting the nominations out of reach for other contenders.
Even before the results started flowing in, Trump was calling for Florida Sen. Marco Rubio, one of his chief rivals, to give up if he didn't win anywhere on Tuesday.
"He has to get out," he told Fox News. "He hasn't won anything."
Rubio, along with Texas Sen. Ted Cruz, is scrambling to block Trump's path to the nomination. Both senators have launched furious verbal attacks on the billionaire businessman in recent days, but some in the party establishment fear the anti-Trump campaign has come too late.
Cruz once saw the Southern states that vote Tuesday as his opportunity to stake his claim to the nomination. Now his campaign's future hinges on a victory in his home state of Texas, the biggest prize up for grabs.
Rubio's goal is even more modest. He's seeking to stay competitive in the delegate count and hoping to pull off a win in his home state of Florida on March 15.
Republicans spent months largely letting Trump go unchallenged, wrongly assuming that his populist appeal with voters would fizzle. Now GOPO leaders are divided between those who pledge to fall in line behind him 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 could foreshadow an extraordinary split in the party this fall.
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 later said he had not understood the interviewer who first raised the question about Duke, and he did repudiate him.
House Speaker Paul Ryan said Tuesday that anyone who wants to be the Republican presidential nominee must reject any racist group or individual.
"When I see something that runs counter to who we are as a party and a country I will speak up. So today I want to be very clear about something: If a person wants to be the nominee of the Republican Party, there can be no evasion and no games. They must reject any group or cause that is built on bigotry," Ryan said.
The disarray among Republicans comes as Clinton appears to be tightening her grip on the Democratic field. She scored a blowout victory over Bernie Sanders in Saturday's South Carolina primary, a contest that underscored her strength with black voters.
Clinton's campaign is hoping that support will continue in Tuesday's contests in several Southern states with large African-American electorates.
She 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 flaming, the finger pointing that is going on the Republican side," she told voters in Springfield, Massachusetts. "It really undermines our fabric as a nation."
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 other states including Minnesota and his home state of Vermont. But Sanders faces tough questions about whether he can rally minorities who are core Democratic voters.
After he voted Tuesday in his hometown of Burlington, Sanders told reporters that if voter turnout is high "we are going to do well. If not, we're probably going to be struggling."
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.
States holding voting contests in both parties are Alabama, Arkansas, Georgia, Massachusetts, Minnesota, Oklahoma, Tennessee, Texas, Vermont and Virginia. Republicans also vote in Alaska and Democrats in Colorado. Democrats also have a contest in American Samoa and for Democrats Abroad.
Colvin reported from Valdosta, Georgia. AP writers Alan Fram and Andrew Taylor contributed to this report.
Follow Julie Pace and Jill Colvin on Twitter at: http://twitter.com/jpaceDC and http://twitter.com/colvinj