NORTH CANTON, Ohio — Weeks of bad press for himself and a sense of urgency among his opponents may finally be taking a toll on Republican presidential front-runner Donald Trump, who heads into Tuesday primaries looking at possibly his worst night of the campaign since he lost Iowa’s caucuses.

The billionaire businessman’s lead has evaporated in polling in Ohio, where Gov. John Kasich has surged. Meanwhile, in Illinois and Missouri, Sen. Ted Cruz is within striking distance.

Mr. Trump still holds a comfortable lead in polling in North Carolina and is poised for a massive win in Florida, which rounds out the Republican contests Tuesday.

But a loss in Ohio, where the winner gets all of the state’s delegates to the nominating convention, could be devastating for Mr. Trump, dooming him to fight it out at the Republican National Convention in July against a party establishment increasingly worried about his candidacy in the November general election.

Mitt Romney, the 2012 Republican presidential nominee, endorsed Mr. Kasich on Monday, saying the Ohio governor is the right candidate to stop Mr. Trump and take the helm of the party.

“You look at this guy and, unlike the other people running, he has a real track record. He has the kind of record that you want in Washington, and that is why I am convinced you are going to do the right thing tomorrow — agreed?” Mr. Romney said as he campaigned with Mr. Kasich in North Canton.

Mr. Trump is much less certain of the outcome than he had been going into previous primaries.

“I hope I win Ohio,” the businessman said as he campaigned in Tampa on Monday, just hours before he jetted back to Youngstown, Ohio, for a last-minute rally to try to stop his bleeding. “If we win Ohio and we win Florida, then everybody agrees it’s over.”

The momentum Mr. Trump had coming out of Super Tuesday and subsequent wins has appeared to slip. He has faced intense scrutiny over his business practices and over clashes between protesters and his supporters at his campaign rallies.

He has canceled a Chicago rally Friday after he deemed protesters a threat to safety, and his rallies since have focused heavily on trying to keep everyone safe — at times overshadowing his effort to get out his message.

The other three major Republican candidates still in the race have blamed Mr. Trump for some of the escalation, saying he is playing to voters’ fears and the clashes are a result of that.

“If we allow the Republican Party and the conservative movement to be defined as anger and frustration, we will not win,” Sen. Marco Rubio said as he campaigned Monday in Florida, with his own political future on the line.

Mr. Trump is poised to trounce the first-term senator in Florida, with polls showing the businessman maintaining a lead of about 20 percentage points.

“Florida is do or die for the Rubio campaign, but it looks like victory may have slipped from his grasp,” said Patrick Murray, director of the independent Monmouth University Polling Institute, whose own pre-election survey Monday showed Mr. Trump up 44 percent to 27 percent over Mr. Rubio.

Mr. Trump continued to show his strength in Florida, picking up the endorsement Monday of Attorney General Pam Bondi, a rising Republican star who said she and the businessman shared an outsider’s approach to trying to shake up government.

“You are speaking loud and clear, and Americans are speaking loud and clear,” Ms. Bondi told Trump supporters in Tampa.

Florida is Mr. Trump’s best state Tuesday, though he also holds a big lead in polling in North Carolina and a significant lead in Missouri — though polling there has been scarce. The latest survey in Illinois, taken by CBS News/YouGov, shows Mr. Trump with a slim 4-point lead over Mr. Cruz.

Ohio is where Mr. Trump has seen the worst slippage, with Mr. Kasich overtaking him in most polling.

Aware of his standing, Mr. Trump took aim at Mr. Kasich on Monday, calling him an “absentee” governor who spent more time campaigning in New Hampshire than in working for his state.

Mr. Trump said Mr. Kasich is “weak on the border” and criticized him for voting for NAFTA as a member of Congress and for supporting the Trans Pacific Partnership agreement, warning it will hurt American workers.

Mr. Trump’s supporters aren’t discouraged at the tougher-than-expected fight.

Janet Riley, 54, from Tampa, blamed the press for the Mr. Trump’s tumble, saying they media dismissed him at first but are going after him now that he has a real chance of winning the nomination.

Bob Walker, 62, from St. Petersburg, doubted the latest numbers showing a Trump slide.

“I would look at who’s paying for the polls. If you’re trying to smear him, like they are in the media, the best way to smear him is to rig poll numbers,” Mr. Walker said as he waited outside the Trump rally Monday afternoon. “You can create a poll to get any answer you want. It’s how you ask the question. They never tell you what the margins of error are.”

He said those polls will be proved wrong Tuesday.

“Oh, he’ll win. He’ll win. No doubt about it,” Mr. Walker said.

Mr. Trump has survived a full barrage of negative attack ads over the past few weeks and marveled Monday at his ability to survive. He said voters deserve the credit.

“The people of this country are smart. They get it,” he said.

• David Sherfinski contributed to this report.

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