REPUBLICAN Donald Trump and Democrat Hillary Clinton will see an opportunity to pull away from their rivals on so-called Super Tuesday, the busiest day of the 2016 primary campaign in the US election.
The contests come at a turbulent moment for the Republicans as they grapple with the prospect of Mr Trump becoming the party's nominee.
His main rivals, Florida Senator Marco Rubio and Texas Senator Ted Cruz, are engaged in a frantic effort to stop the billionaire property magnate, but it was unclear whether they had made their move too late.
Tensions boiled over during a Trump rally on Monday in Virginia, where he was repeatedly disrupted by demonstrators, including 20 or more chanting: "Black lives matter."
At another point, he asked a protester "Are you from Mexico?" after he was interrupted during remarks about immigration. He ordered several people to be removed, then cast himself as a unifying political force.
"Believe it or not, we're going to unify this country," he said.
Like Mr Trump, Mrs Clinton has won three of the four early voting contests, including in South Carolina on Saturday.
Her victory over Vermont Senator Bernie Sanders 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.
Mrs Clinton has chosen to focus on Mr Trump in recent days. She is 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 Massachusetts. "It really undermines our fabric as a nation."
Mr Sanders, who has energised young voters with his call for a political revolution, was seeking to pick up victories in states including Minnesota and Vermont.
But he faces tough questions about whether he can rally minorities, who are core Democratic voters. Still, he has the resources to stay in the race through the last primaries in June, with his campaign announcing it had raised more than 41 million US dollars (£29 million) in February.
Democrats will vote in 11 states and American Samoa on Super Tuesday, with 865 delegates up for grabs. It will take 1,237 delegates to secure the nomination at the party's national convention in July.
Republicans will vote in 11 states, with 595 delegates at stake - nearly half the 1,237 delegates needed to gain the nomination at the party's convention, also in July.
States holding voting contests in both parties are Alabama, Arkansas, Georgia, Massachusetts, Minnesota, Oklahoma, Tennessee, Texas, Vermont and Virginia. Republicans vote in Alaska and Democrats in Colorado.
Mr Trump was seeking to sweep the South, which would be a massive blow for Mr Cruz. The Texas senator, a favourite of the region's social conservatives and evangelical Christians, expected the South to be his strength, but now is simply hoping for a victory in his home state.
Mr Rubio's goal on Super Tuesday is even more modest. He is 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 the Republicans' best chance to win in a general election and has received a flood of endorsements from party officials after other more mainstream candidates dropped out. But he has failed to win a state so far.
Republicans spent months largely letting Mr Trump go unchallenged, wrongly assuming that his populist appeal with voters would fizzle.
An AP survey of Republican senators and governors across the country showed just under half of respondents would not commit to backing Mr Trump if he were the nominee. Their reluctance foreshadowed a potentially extraordinary split in the party this autumn.
The worries among Republicans appeared to grow after Mr Trump briefly refused to disavow former white supremacist leader David Duke during a television interview. Mr Trump said he had not understood the interviewer who first raised the question, and he did later repudiate Mr Duke.
