Five states hold their Republican and Democratic primaries on Tuesday: Florida, Illinois, Missouri, North Carolina and Ohio.
With so much on the line, the so-called Super Tuesday 3 is shaping up to be a crucial night in the 2016 presidential race.
Raising the stakes on the Republican side is that Florida, with its 99 delegates and Ohio, which awards 66 delegates, are winner-take-all contests.
That means for the two home-state candidates, Ohio Gov. John Kasich
and Florida Sen. Marco Rubio
, anything short of wins in their own backyards could leave them so far behind in the delegate count that they'd face intense pressure to end their campaigns.
A sweep by Trump would mean all that would be left are extraordinary measures -- like a contested convention. And even that could be out of reach.
So in Ohio, Kasich's friends in the GOP establishment are pulling out all the stops.
Former House Speaker John Boehner endorsed his fellow Ohioan at a Butler County Republican Party event. He said the two had spent 18 years together in Congress, and he'd already voted early for Kasich.
"He's my friend," Boehner said.
, the 2012 Republican nominee who has recently become one of Trump's chief antagonists, will hit the campaign trail alongside Kasich
in the run-up to Ohio's primary.
Polls show Kasich and Trump close in the Buckeye State.
And the contest in Ohio comes just a week after Kasich finished third in Michigan -- a state with similar demographics, and where he spent a good bit of time campaigning.
In Florida, the situation is more dire for Rubio
, who polls show trailing Trump by a 2-to-1 margin.
His campaign has long argued that the more moderate states that vote later in the process is where Rubio would have his best showings. But a loss in Florida would put Rubio at risk
of seeing his fundraising grind to a halt as he falls further behind in the delegate count.
Rubio spent Sunday making the case that Tuesday's contests are all about stopping Trump -- and arguing that blocking Trump is still possible.
"I think we're having a battle to define conservatism in the Republican Party," he said on ABC's "This Week."
"I do not want the Republican Party or the conservative movement to be defined by what I'm seeing out of Donald Trump's campaign."
Rubio said the mathematical path to the nomination, even for Trump, remains daunting.
"Despite all this noise that's out there, he needs 60% of the delegates from this point forward in order to be the nominee. Ted Cruz by the way needs 75% of the remaining delegates to be the nominee. That's the real math," Rubio said. "I at the end of the day do not believe that Donald Trump will be our nominee and I'm going to do everything possible to keep that from happening and to give the party a choice in me, someone that people aren't going to have to be asked that question about."
Texas Sen. Ted Cruz
doesn't lead the polls in any of the five states, but Tuesday's contests will once again test the strength of his campaign's data and organizational capabilities.
There are opportunities for Cruz to rack up delegates -- particularly in down-state Illinois congressional districts, Missouri and North Carolina. His campaign has been carefully calibrating his schedule to seize on those opportunities -- which means he'll try to stay close to Trump in the delegate count even without winning a state.
Cruz's argument is about electability. He said on ABC's "This Week" that a Trump nomination would be "a disaster for Republicans, for conservatives. I think it's a disaster for the country because if Donald is the nominee, it makes it much, much more likely that Hillary Clinton wins the general."
Trump said Sunday on CNN's "State of the Union" that he's not sweating his GOP rivals
"They are losing big league," he said.
He added, "In Florida, we have a man, Marco Rubio, who doesn't even show up to vote in the U.S. Senate. He's a disgrace. He's weak, very weak on illegal immigration, wants to give amnesty to everybody. He's a person that I don't think he could be elected dogcatcher in Florida, frankly."
Of Kasich, Trump said: "If you look at Ohio, we have a man that voted for NAFTA. NAFTA has destroyed Ohio."
Clinton is all but assured of finishing her sweep of the South by picking up wins in Florida and North Carolina.
The real battleground will be the Midwest. Sanders will try to replicate his stunning victory in Michigan last week by winning similar big, manufacturing-heavy, states: Illinois, Missouri and Ohio.
It's an especially important set of contests because Clinton's allies hope she can deliver a knockout blow before the race shifts west, to states where Sanders expects he'll be more formidable.
As they made their final pitches in Ohio at a CNN town hall Sunday night, both candidates focused on how they'd take on Trump
Clinton played up her tenure as secretary of state in arguing that Trump's incendiary rhetoric undermines "our standing in the world."
"I'm having foreign leaders ask if they can endorse me and stop Donald Trump," she said.
She reminded the audience that so far in the primary process, she's received more votes than anyone else -- including Trump and Sanders.
And she said she's the best bet to stand up to Trump in the general election because the Republicans who have "been after me for 25 years" have already thrown the entire book at her.
"In the course of dealing with all of this incoming fire from them, I have developed a pretty thick skin. I am not new to the national arena, and I think whoever goes up against Donald Trump better be ready," Clinton said.
Sanders pointed to polls that show him leading Trump head-to-head nationally. And he said his ability to bring young people into the political process can counteract the GOP's ability to "win when voter turnout is low."
"The excitement and the energy for large voter turnouts is with the Sanders campaign," he said.
The senator argued he'd "expose" Trump, citing the businessman's opposition to a minimum wage increase and his leadership of "the so-called 'birther' movement."
"The American public is not going to elect a president ... insulting virtually everybody who is not like Donald Trump. Thank God most people are not like Donald Trump," he said.
While Trump has dominated the overall race, the issue that's received the most focus in recent days in the Democratic contest is trade.
Sanders has been relentless in hammering Clinton for supporting the North American Free Trade Agreement, which was signed into law by Bill Clinton, and for expanded trade with China.
He has cast her as slow to announce her opposition to the Trans-Pacific Partnership, a 12-country mega-deal that Sanders opposed from the beginning of President Barack Obama's administration's negotiations.
Clinton, meanwhile, is focusing her trade argument on specifics. She promised to beef up anti-dumping trade taxes on foreign steel imports to help a key industry in Ohio. And she says she'd appoint a "trade prosecutor" who would challenge unfair foreign practices on the government's behalf -- rather than waiting on a U.S. company to do it.
She sought to bolster herself on the issue -- revealed as a vulnerability in Michigan, where nearly three in five Democratic voters said they see trade deals
as hurting the U.S. economy, and a majority of those voters supported Sanders, exit polls showed -- during the town hall.
"I know there has been a lot of discussion in the past week or so about trade. And I would like to take the opportunity tonight to set the record straight," Clinton said. "To every worker in Ohio, every worker across America, let me say this: If I am fortunate enough to be your president I will stand with you, I will have your back and I will stop dead in its tracks any trade deal that hurts America and American workers."