Columbus, Ohio (CNN)If Republicans are going to slow Donald Trump's march to the nomination, Tuesday's contests could be their last chance to do so at the ballot box, rather than in a contested convention.
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Super Tuesday 3: What to watch
And if Bernie Sanders is going to catch Hillary Clinton's Democratic delegate lead, he'll need to pass a big test in Midwestern battleground states.
Voters go to the polls in Florida, Illinois, Missouri, North Carolina and Ohio on Tuesday. Here's what to watch in those contests:
Once again, Trump has dominated the discussion headed into a big election day -- with news cycles in the closing days before the third straight Super Tuesday focused on violence at his campaign rallies. He canceled one in Chicago amid security fears; he said he would look into paying the legal fees of a supporter who is charged with sucker-punching a protestor in North Carolina; he called for protesters who demonstrate at his rallies to be arrested.
Trump has so far managed to avoid any damage from the controversies that have engulfed his campaign from the start. But Tuesday provides another test -- with states like Ohio threatening to chip away at his inevitability as the GOP nominee.
It's telling for Republicans who are dead-set on stopping Trump that their hopes Tuesday are resting with two candidates in Florida Sen. Marco Rubio and Ohio Gov. John Kasich who haven't shown much ability to win contests.
That demonstrates the race's evolution: Instead of promoting one of the other candidates, it's all about keeping Trump from amassing the 1,237 delegates he needs to win the party's nomination outright.
But polls show Trump either ahead or within striking distance in all five states -- and a clean sweep could make it next to impossible for any of his rivals, even Cruz, to catch his delegate number by the convention.
If he wins them all, Republicans will be down to a last hope of somehow denying Trump the delegates he needs and driving the race into Cleveland in a contested convention.
Trump could try to hammer that point home at his Mar-A-Lago club in Palm Beach, Florida, on Tuesday night. He'll hold another election-night news conference -- guaranteeing wall-to-wall coverage -- where he could try to drive his foes out of the race.
The two most important Republican contests to watch are the winner-take-all states: 99-delegate Florida and 66-delegate Ohio.
Both have home-state favorites in Rubio and Kasich -- and both are fighting to save their campaigns by beating Trump.
Neither has left much doubt: It's win or stay home.
Rubio and Kasich have justified their decisions to stay in the race by pointing to their delegate-rich home states as turning points -- even though they haven't racked up many wins on the way.
So far, Rubio has wins in Minnesota, Puerto Rico and the District of Columbia to show for himself. Kasich has strong second-place showings in New Hampshire and Vermont, but no victories.
The situation is especially dire for Rubio, who has trailed Trump by a 2-to-1 margin in recent polls of his home state's likely GOP primary voters.
Backers have been patient with the 44-year-old Florida senator, buoyed by the political talent he has flashed -- especially on the debate stage. And there is a sense that he'd perform better when the race reaches more moderate coastal states. But a poor performance in his home state may mean the end for his 2016 ambitions.
Sanders, the Vermont senator who stunned the political world by upsetting Clinton in Michigan a week ago, still trails in the overall delegate race.
But he's in better shape than Clinton's allies expected him to be at this stage. March 15 was supposed to be the day he was knocked out once and for all. Now, thanks to Michigan, Sanders is seriously competing in three states.
He's betting his criticism of trade deals will hurt Clinton in Illinois, Missouri and Ohio.
It all has Clinton, he said Sunday, "getting a little bit nervous" about the Democratic presidential race.
"I think she understands that the momentum in this campaign and the energy is with us, and that we have a good chance to win a number of states on Tuesday," Sanders said in an interview with CNN's Jake Tapper on "State of the Union."
Sanders has always said the campaign will shift in his favor starting Wednesday, when the race moves west. A win in any of Tuesday's states would have been a surprise two weeks ago -- but a win in multiple states could give him a critical boost of momentum just as the race moves onto friendlier terrain.
There's a reason Sanders is behind in the delegate count, though: Clinton may be about to run the table in the South.
She's won South Carolina, Texas and everything in between already, and she is expected to finish off the sweep with wins in Florida and North Carolina.
Because Democratic delegates are awarded on a proportional basis, she could win those two, lose the three Midwestern states, and still gain more delegates than Sanders on the night.
And that's the point Clinton's campaign will make: Even if it's going to be a long slog to the Democratic National Convention in Philadelphia, Clinton is doing everything necessary to win it -- no matter which way something as fleeting as momentum is going in a given week.
If Clinton can defeat Sanders everywhere, though, it would show she's cleared a key hurdle with younger and white voters -- and has done so in swing states that are critical in November.
Texas Sen. Ted Cruz doesn't lead the polls in any of the five states voting Tuesday, but the contests will 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.
That's Cruz's first goal on Tuesday, and it leans into the second: drive Rubio and Kasich out of the race.
"It will be absolutely clear to everyone that this is a two-man race," Cruz said told reporters Monday in Peoria, Illinois. "I think after tomorrow it will be officially a two-man race. Because no other candidate has any plausible path to 1,237."
Establishment Republicans may never like Cruz. But the violence that has erupted at Trump events in recent days could rally them to Cruz's cause nonetheless, with the hope that the Texas senator wouldn't cause the party long-term damage or embarrassment.
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."