Democrat Hillary Clinton aims to build an impregnable lead on "Super Tuesday," the most consequential day of the presidential nominations calendar, while Republicans struggle to derail their insurgent and controversial frontrunner Donald Trump.
With barely 24 hours before the big day, Clinton and Trump are well positioned to secure the lion's share of the delegate bonanza in the 11 states voting in each party's primaries.
Clinton, riding high after thrashing rival Bernie Sanders Saturday in South Carolina, could come close to staking her claim to the nomination on March 1 when the race goes national, after a string of smaller but important single-state contests.
Trump, whose brash and incendiary campaign has turned American politics on its head, has a political target on his back, with mainstream favorite Marco Rubio assailing the real estate mogul during every campaign stop now.
Super Tuesday will unquestionably be a gut check for the Republican Party.
It will also test whether Rubio's newfound aggression against Trump - the 44-year-old senator has attacked his business dealings, temperament, looks, age and policy platforms in recent days - will affect voters.
"We can't nominate someone who's going to lose," Rubio said at a campaign stop in Purcellville, Virginia.
"Never Trump!" an audience member shouted out.
Trump's extraordinary bombast during the campaign, including calling some Mexican immigrants "rapists" 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 persistently attacks the establishment.
In the latest controversy, Trump came under withering criticism for refusing to disavow the support of David Duke, a white supremacist who once led the Ku Klux Klan.
"I don't know what group you're talking about. You wouldn't want me to condemn a group that I know nothing about. I'd have to look," Trump told CNN's "State of the Union."
His comments drew fierce criticism from across the political spectrum.
"We cannot be a party that nominates someone that refuses to condemn white supremacists and the Ku Klux Klan," said Rubio, seen as the Republican best positioned to oust Trump.
"Not only is that wrong, it makes him unelectable."
Republican rivals Senator Ted Cruz and Ohio Governor John Kasich also criticized Trump, as did Clinton and Sanders.
Sanders, a self-declared democratic socialist, wrote: "America's first black president cannot and will not be succeeded by a hatemonger who refuses to condemn the KKK."
In a rare sign of agreement between the rivals, Clinton re-tweeted Sanders's comment.
Controversy also swirled over Trump's retweet of a quote attributed to late Italian leader Benito Mussolini: "It is better to live one day as a lion than 100 years as a sheep."
Trump shrugged off charges that he is a supporter of the fascist.
"I want to be associated with interesting quotes," he told NBC's "Meet the Press." "Hey, it got your attention, didn't it?"
If Trump sweeps the South, where many of the Super Tuesday races are taking place, it could be lights out for his Republican challengers.
"There is no doubt that if Donald steamrolls through Super Tuesday, wins everywhere with big margins, that he may well be unstoppable," Cruz acknowledged on CBS's "Face the Nation."
Cruz is from Texas, the largest prize in Tuesday's voting, and he is banking on winning his home state.
He hopes his brand of arch-conservatism will also win the day in several southern states with significant evangelical Christian voters.
In Alabama, it was Trump who held sway, hosting a huge rally where he estimated the stadium crowd at 32,000.
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.
But even as the establishment scrambles furiously to block Trump's path, some members of the Republican inner circle have thrown their support behind The Donald.
New Jersey Governor Chris Christie endorsed him this past week, while Senator Jeff Sessions, a conservative stalwart who has intensely opposed President Barack Obama's executive orders on immigration, announced Sunday he is backing Trump.
Clinton, fresh from her commanding defeat of Sanders in South Carolina, by 73.5 percent to 26 percent, felt confident enough to skip the Sunday talk shows, a typical post-primary stop for candidates in the thick of the presidential race.
Instead, she headed straight to Tennessee, where she made some closing arguments in her bid to become America's first female commander in chief.
"America is great right now. What we need is to be whole," she said at a medical college, playing off of Trump's campaign slogan, "Make America Great Again."
On Monday, she and Trump will each hold events in Virginia.