Donald Trump and Hillary Clinton set sights on Super Tuesday wins
Republican Donald Trump and Democrat Hillary Clinton are looking to pull away from their rivals on Super Tuesday, with almost a dozen state contests across the country key to accelerating their march towards the general election.
It is busiest day of the 2016 primary campaign, with a quarter of Americans having their say.
The contests come at a turbulent moment for 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 real estate magnate, but it was unclear whether they had made their move too late.
On Tuesday Mr Trump said Rubio should drop out of the race if he doesn't win a single Super Tuesday state. "He has to get out. He hasn't won anything," he told Fox News.
Like Mr Trump, Mrs Clinton has won three of the four early voting contests, including in South Carolina on Saturday. Her victory over Vermont Sen Bernie Sanders there was due to support from black voters, putting her in position for a strong showing in several Southern states with large African-American electorates voting on Tuesday.
Mrs Clinton is casting herself as a civil alternative to the insults and bullying that have consumed the Republican race, which she said has "turned into a kind of one-upmanship on insulting, and I don't think that's appropriate in a presidential campaign".
Of Mr Trump, she said that "obviously, he's done very well".
Mr Trump pushed back against criticism by suggesting that he's even attracting Democrats to the Republican Party. "I can tell you the one person Hillary Clinton doesn't want to run against is me," he told ABC.
He told a rally later that Mrs Clinton "does not have the strength or the stamina to be president".
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.
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 on Tuesday 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 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 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.