FAIRFAX, UNITED STATES
Millions of Americans were preparing to vote in a dozen states on "Super Tuesday," with presidential frontrunners Hillary Clinton and Donald Trump poised for a big day that could leave rivals scrambling to stay in the race.
With just hours to go before polls open, the Republican and Democratic frontrunners fended off rivals and made last-ditch appeals to supporters ahead of what may be the most consequential day of voting in the 2016 primaries.
If they win big, it could spell doom for their challengers.
Rival Senators Marco Rubio and Ted Cruz were trying frantically to halt Trump's march toward the nomination, seeking to unite the party against the man they see as a non-conservative political interloper.
Clinton meanwhile was riding high after thrashing rival Bernie Sanders in South Carolina over the weekend, securing an astronomical 86 per cent of the African-American vote in her third win in four contests.
Should she win black voters by similar margins in places like Alabama, Georgia and Virginia, she should dominate there to become once again the inevitable candidate.
That was her status at the start of the campaign — before the rise of Sanders, a self-described democratic socialist.
But she was leaving nothing to chance, traveling to multiple states Monday to urge a strong turnout.
She also took aim at the increasingly hostile campaign rhetoric on the Republican side led by the brash real estate mogul Trump.
"I really regret the language being used by Republicans. Scapegoating people, finger-pointing, blaming. That is not how we should behave toward one another," she told hundreds at a university in Fairfax, Virginia.
"We're going to demonstrate starting tomorrow on Super Tuesday, there's a different path that Americans ought to take."
Trump's incendiary campaign has infuriated Republican rivals, including mainstream favourite Rubio who has intensified his personal attacks and stressed Trump would have trouble in a general election.
The Florida senator warned supporters in Tennessee that US media and Democratic groups will jump on Trump "like the hounds of hell" if he wins the nomination.
In a sign of the tension surrounding Trump's campaign, his Monday rally Radford University in southwest Virginia was repeatedly disrupted by protesters, and a Secret Service agent scuffled violently with a Time magazine photographer.
But Trump is clearly in the driver's seat. He is leading in polling in at least eight of the 11 Super Tuesday states.
And a new CNN/ORC poll shows the billionaire expanding his lead nationally, earning a stunning 49 per cent support compared to second place Rubio at 16 per cent.
Cruz of Texas is third, at 15 per cent, followed by retired neurosurgeon Ben Carson at 10 per cent and Ohio Governor John Kasich at six per cent.
Trump punched back against Rubio, calling him "Little Marco," mocking him for sweating on the campaign trail and warning Rubio could not stand up to strong men like Russian President Vladimir Putin.
'PARTY OF LINCOLN'
Trump's inflammatory rhetoric, including accusing Mexico of sending "rapists" across the border, mocking women and the disabled, and urging a ban on Muslims entering the country, would have been the undoing of a normal candidate.
But the 2016 cycle has been far from normal, with a furious electorate keen to back an outsider who scorns the establishment.
"I'm representing a lot of anger out there," Trump told CNN.
"We're not angry people, but we're angry at the way this country's being run."
In the latest controversy, Trump came under withering criticism for not immediately disavowing the support of David Duke, who once led the Ku Klux Klan.
Rubio said Trump's failure to promptly repudiate Duke, who has expressed support for Trump, makes him "unelectable."
Republican 2012 flagbearer Mitt Romney joined the chorus of outrage, tweeting that Trump's "coddling of repugnant bigotry is not in the character of America".
Some conservatives have spoken up to say they will shun Trump if he is the nominee.
"This is the party of (1860s president) Abraham Lincoln," said Senator Ben Sasse, accusing Trump of being a non-conservative plotting a "hostile takeover" of the party.
Trump supporters, Sasse told MSNBC, "need to recognise that there are a whole bunch of other people who say, if this becomes the David Duke/Donald Trump party, there are a lot of us who are out."
If Trump sweeps the South, where many of the Super Tuesday races are taking place, it could be lights out for his Republican challengers.
Texas is the largest prize Tuesday, and Cruz is banking on winning his home state. He trails in nearly all other Super Tuesday states.
Almost 600 Republican delegates are up for grabs Tuesday, nearly half the 1,237 needed to secure the nomination.
Some 865 Democratic delegates are at stake, 36 per cent of those needed to win.