Donald Trump and Hillary Clinton face a potentially decisive day on Tuesday with contests in five states that could transform them from front-runners to likely nominees in the Republican and Democratic primary campaigns.
Florida Sen. Marco Rubio and Ohio Gov. John Kasich were fighting for their political futures, desperate for wins in their home states to keep their White House hopes alive and complicate Trump's path to the nomination. Clinton is hoping to keep rival Bernie Sanders from building new momentum in the Midwest.
While Florida and Ohio are the biggest prizes, Missouri, Illinois and North Carolina are also awarding a large cache of delegates to the parties' national nominating conventions.
Trump enters Tuesday's primaries embroiled in one of the biggest controversies of his contentious campaign. The Republican front-runner has encouraged supporters to physically confront protesters at his events and is now facing criticism for encouraging violence after skirmishes broke out at a rally last week in Chicago.
During an event Monday in Tampa, Trump was interrupted intermittently by protesters, some of whom were forcibly removed. Trump said he didn't want to "ruin somebody's life, but do we prosecute somebody like that?"
Trump holds a comfortable lead in the Republican delegate count. If he sweeps Tuesday's contests, he would cross an important threshold by collecting more than 50 percent of the delegates awarded so far.
Kasich and Rubio hoped for victories in their home-state primaries — winner-take-all contests with 99 delegates at state in Florida and 66 in Ohio — to give them a boost, but a loss could end their candidacies. Polls show Rubio trailing Trump by a wide margin in Florida, while Kasich holds a narrow lead in Ohio.
That makes Ohio the key state in determining whether Trump puts himself on a path to winning the nomination by the end of the primary season on July 7. A loss means the race continues on an uncertain path, possibly resulting in a contested convention.
Trump's closest competition has come from Texas Sen. Ted Cruz, who has defeated the businessman in seven states.
Among Democrats, Clinton urged Democrats to unite behind her bid to focus on the far bigger threat posed by Trump. At a Monday rally in Chicago, Clinton said, "We have the way forward to be able to start talking about not only unifying the Democratic Party but unifying our country."
But the former secretary of state continues to face persistent competition from Vermont Sen. Bernie Sanders who breathed new life into his campaign with a surprising victory last week in Michigan. Sanders' advisers see the potential for success in Missouri, Illinois and Ohio which would give his campaign fresh momentum in what has become an uphill battle for the nomination.
If Clinton comes out of Tuesday's contests with decisive wins in several states and further pad her delegate lead, it would be difficult for Sanders to catcher her because all the state Democratic nominating contests allot their delegates proportionally.
The vibe at Trump's events has deepened the angst over his candidacy in some Republican circles. Rubio and Kasich have suggested they might not be able to support Trump if he's the nominee, an extraordinary stance for intraparty rivals. House Speaker Paul Ryan has also taken lightly veiled shots at the businessman, who has denied playing any role in encouraging violence against protesters.
"I think the candidates need to take responsibility for the environment at their events," Ryan said during an interview Monday with WRJN, a radio station in Racine, Wisconsin. "There is never an excuse for condoning violence, or even a culture that presupposes it."
Kasich spent Monday campaigning in his home state alongside Mitt Romney, the 2012 Republican nominee and a fierce critic of Trump.t a Kasich event in North Canton, Ohio. While Romney has not endorsed Kasich, he's said he'll do whatever is needed to help all of Trump's rivals.
Rubio, despite having the backing of numerous Republican elected officials, appears to have slipped in recent public polls in Florida. The senator tried to stay upbeat Monday, perhaps his final full day of campaigning in the 2016 race.
"Tomorrow's the day where we are going to shock the country," Rubio said during a stop in Jacksonville.
Trump went into Tuesday's contests with 460 delegates to 370 for Cruz, 163 for Rubio and 63 for Kasich. It takes 1,237 delegates to win the Republican nomination.
In the Democratic race, Sanders reprised a theme that helped propel his Michigan win. On Monday, Sanders, a self-described democratic socialist, pounded Clinton's past support for trade deals such as the North American Free Trade Agreement with Canada and Mexico. He's escalated his criticism in recent days, hoping to undercut her edge among minorities and expand his advantage with white working-class voters.
"When it came down whether you stand with corporate America, the people who wrote these agreements, or whether you stand with the working people of this country, I proudly stood with the workers," Sanders said in Youngstown, Ohio. "Secretary Clinton stood with the big money interests."
Clinton's team is attempting to tamp down expectations for Tuesday night, stressing that the race remains close in the Midwest. Still, she's eying the general election and escalating her attacks on Trump, saying he's "inciting mob violence" at his rallies.
"I do hold him responsible," she said in an interview with MSNBC. "He's been building this incitement, he's been leading crowds in jeering protesters"
Clinton headed into Tuesday's primary with 768 pledged delegates compared to 554 for Sanders, according to a count by The Associated Press. Including superdelegates, elected officials and party insiders who are free to choose any candidate, Clinton has 1,235 total delegates, more than half the amount needed to clinch the nomination, while Sanders has 580.