WEST CHESTER, Ohio — Unsettled by images of violence and ugliness at Donald J. Trump's rallies, the Republican leaders pushing to stop him are desperately targeting voters in the five big states that vote on Tuesday.
The weekend of disturbances at Mr. Trump's rallies injected more unease into the race, and the appeals of his rivals turned emotional and urgent. On Sunday, Senator Marco Rubio of Florida warned that a Trump White House would exploit hatred and stir violence. "Imagine what that means for the country," Mr. Rubio said. "Imagine the tone it sets for our culture."
Ohio has emerged as a critical contest, the one large state voting this week where Mr. Trump appears vulnerable. A victory here by Gov. John Kasich would complicate Mr. Trump's attempt to gather a majority of delegates needed for the nomination.
On the Democratic side, Senator Bernie Sanders of Vermont hoped for a repeat of his stunning upset over Hillary Clinton last week in Michigan, as he hopscotched the Midwest to push his central message that American workers have suffered too much under trade deals.
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Although the states that voted two weeks ago on "Super Tuesday" received much attention, the contests this week are potentially even more important.
A total of 424 delegates are up for grabs on the Republican side, and, for the first time, states can award all of their delegates to the winner of the popular vote.
With two polls in recent days showing Mr. Kasich with an edge over Mr. Trump in Ohio, whose 66 delegates will be awarded winner-take-all on Tuesday, Mr. Trump made multiple stops in the state over the weekend and added a last-minute return visit on Monday, canceling an appearance in Florida, where he has a strong lead.
Mr. Trump held two rallies on Sunday that were mainly free of protesters' disturbances.
The unrest at Mr. Trump's rallies has made the race more tense. Mr. Trump blamed Mr. Sanders's supporters for shutting down a Trump rally on Friday in Chicago. But he has been sympathetic to his supporters who have become violent. On Sunday, Mr. Trump said he would consider paying the legal costs for a man who was charged last week with assaulting a protester being escorted from a rally in Fayetteville, N.C.
In West Chester, Ohio, on Sunday, Mr. Trump spoke on a raised platform surrounded by a cordon of uniformed and plainclothes officers.
If Mr. Kasich is defeated in Ohio, he has indicated he will exit the race. Mr. Rubio is likely to withdraw, too, if he loses his home state, Florida. That would leave Mr. Trump facing Senator Ted Cruz of Texas in a head-to-head competition, and polls show Mr. Trump is weaker in that matchup than in a multicandidate field.
Mr. Cruz, who is well behind in the polls in Ohio and Florida, focused on the other three states voting on Tuesday: Missouri, North Carolina and Illinois, where he planned to hold five events on Monday.
Mr. Cruz said he was "neck and neck" with Mr. Trump in the three states.
In the Democratic campaign, aides to Mrs. Clinton predicted tight races in the Midwestern states, where economically struggling voters have gravitated to Mr. Sanders. They were optimistic about winning Florida and North Carolina, where black, Hispanic and older voters were expected to give Mrs. Clinton a healthy edge in the race for delegates.
Mr. Sanders has linked Mrs. Clinton, in speeches and a television ad, to Rahm Emanuel, the mayor of Chicago, whose record on policing and fights with public-sector unions are unpopular with progressive voters.
In Missouri on Sunday, Mr. Sanders told a crowd that he had, "from Day 1, been a leader in opposition to these disastrous trade policies." He added, "Secretary Clinton has supported virtually all of them."
With Mr. Sanders's ads attacking her support of global trade deals, Mrs. Clinton has emphasized her proposals to lift employment while criticizing the Vermont senator for being short on specific plans.
Speaking at a factory in Youngstown, Ohio, over the weekend, Mrs. Clinton vowed to end steel-dumping practices by China and other nations that undermine the economy in the industrial Mahoning Valley. "I have always been committed to bringing back manufacturing," she said. "And I'm the only candidate, on either side, who actually has a plan to do that."
With Ohio a perennial swing state in presidential elections, and its racially and economically diverse voters fiercely fought over, it was no surprise that candidates converged here in the final hours.
Mr. Trump attacked Mr. Kasich for his support of the Common Core education standards, for a private-sector job at Lehman Brothers, whose 2008 collapse Mr. Trump said "almost destroyed the world," and for supporting trade deals that he blamed for harming the manufacturing sector.
Mr. Kasich accused Mr. Trump of creating "a toxic atmosphere" and said Mr. Trump was on the verge of disqualifying himself to represent the party because of his divisive speech.
The world's eyes were glued to the scenes of mayhem and discord Mr. Trump has created, Mr. Kasich said.
"Our enemies are going to take advantage of them," Mr. Kasich told voters near Cleveland. "Our friends are scratching their heads saying, 'What the heck is happening in America?' "
Mr. Kasich, who has boasted of bringing 400,000 jobs to Ohio and has visited businesses that opened under his watch, has marshaled nearly the entire elected Republican apparatus of Ohio.
On Monday, Mitt Romney, the 2012 Republican nominee who has been an outspoken critic of Mr. Trump, will appear with Mr. Kasich at two Ohio rallies.
Matt Borges, chairman of the state Republicans, warned that if Mr. Trump was the nominee, his divisiveness would cost Republicans the state in the general election. "If we don't carry Ohio in the fall, we don't elect a Republican to the White House," he said.
Mr. Trump's rally here was in West Chester, the hometown of John Boehner, the former House speaker who resigned last year out of frustration with the no-compromise wing of House Republicans. In a rare public appearance, Mr. Boehner endorsed Mr. Kasich at a county Republican dinner in West Chester on Saturday. The Trump rally on Sunday, just a few miles away, was attended by many hundreds more people.
"Kasich's done a great job with Ohio," said Doug Mason, a concrete laborer, but he still planned to vote for Mr. Trump. "I think Trump will be John Kasich on steroids."
The town-hall-style event for Mr. Trump was most notable for his response to a man who said it was important "for a lot of veterans in Ohio" that Mr. Trump clarify a comment, made last year, that former prisoners of war like Senator John McCain were not heroes.
"Oh no, no, no," Mr. Trump said. "They are real heroes."
— Reporting was contributed by Jeremy W. Peters from Largo, Fla.; Yamiche Alcindor from Affton, Mo.; Amy Chozick from Cleveland; Thomas Kaplan from Strongsville, Ohio; Matt Flegenheimer from St. Louis; Jonathan Martin from Washington; and Mitch Smith from Bloomington, Ill.