White House hopefuls Donald Trump and Hillary Clinton are poised to pass a point of no return on Super Tuesday, if as expected they outrun their rivals on the biggest voting day of the primary season.
With just hours to go before polls open, the Republican and Democratic frontrunners fended off rivals and appealed to supporters in a dozen states, a day before they head to polls.
If they win big Tuesday night as the presidential nomination race broadens following a string of statewide votes, it could spell doom for their challengers.
Clinton was riding high after thrashing rival Bernie Sanders Saturday in South Carolina, but she was leaving nothing to chance, traveling to multiple states to urge a strong turnout.
"I need your help to go and vote tomorrow, to bring people to vote with you," she implored to a crowd in Springfield, Massachusetts on Monday.
She also took aim at the increasingly hostile campaign rhetoric on the Republican side led by the brash real estate mogul Trump.
"What we can't let happen is the scapegoating, the blaming, the finger-pointing that is going on on the Republican side, which not only sets a bad example," she said. "It really undermines our fabric as a nation."
Trump, whose incendiary campaign has turned American politics on its head, has a political target on his back, with mainstream favorite Marco Rubio intensifying his personal attacks and stressing Trump would have serious weaknesses in a general election.
The Florida senator warned supporters in Tennessee that US media and critics will jump on Trump "like the hounds of hell" if he wins the nomination.
"They'll shred him to pieces and then get Hillary Clinton elected," Rubio said, insisting that he is better positioned to defeat Clinton.
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 released Monday shows the billionaire expanding his lead nationally, earning a stunning 49 percent support compared to second place Rubio at 16 percent.
Fellow first-term Senator Ted Cruz of Texas is third, at 15 percent, followed by retired neurosurgeon Ben Carson at 10 percent and Ohio Governor John Kasich at six percent.
Super Tuesday will unquestionably be a gut check for the Republican Party.
It will also test whether Rubio's newfound aggressiveness toward Trump -- the 44-year-old senator has attacked his business dealings, temperament, age, policy platforms and hairstyle in recent days -- will have an impact on voters.
Trump is "the Bernie Madoff of American politics" conducting a political "Ponzi scheme," Rubio told Fox News, referencing the former investment advisor jailed for life for committing the largest financial fraud in US history.
"We're going to pull off his mask," Rubio added.
Trump's inflammatory rhetoric during the campaign, accusing Mexico of sending "rapists" and criminals across the border and urging a ban on Muslims entering the country, would have been the undoing of a normal candidate.
But all signs show 2016 is far from normal, with a fiercely angry electorate keen to back an outsider who scornfully attacks the establishment.
In the latest controversy, Trump came under withering criticism from Republican and Democratic candidates alike for not immediately disavowing the support of David Duke, a white supremacist who once led the Ku Klux Klan.
Rubio said Trump's failure to immediately repudiate Duke, who has expressed support for Trump, makes him "unelectable."
The party's flagbearer in 2012, Mitt Romney, joined the chorus of outrage, tweeting: "A disqualifying & disgusting response by @realDonaldTrump to the KKK. His coddling of repugnant bigotry is not in the character of America."
Trump sought to put the issue behind him Monday, blaming it on a "very bad ear piece" that prevented him from accurately hearing the question about Duke.
"I disavowed David Duke. All weekend long on Facebook and Twitter and, obviously, it's never enough. Ridiculous," he told NBC.
If Trump sweeps the South, where many of the Super Tuesday races are taking place, it could be lights out for his Republican challengers. Even rival Cruz said that would make Trump "unstoppable."
Arch-conservative Cruz hails from Texas, the largest prize in Tuesday's voting, and he is banking on winning his home state. But he trails in every other Super Tuesday state but Arkansas.
Nearly 600 Republican delegates are up for grabs Tuesday, close to half of the 1,237 delegates needed to secure the nomination.
A similarly substantial number of Democratic delegates are at stake.
Clinton is ahead in all Tuesday states except Sanders's home state of Vermont.
But Sanders, a self-described democratic socialist, is proving resilient. His campaign announced Monday that it has raised $36 million in February, a record for his campaign.