Presidential candidates gear up for crucial primaries

WASHINGTON, D.C. -- Coming up is one the most critical primaries yet in the race for the White House, with contests Tuesday in five big states. And, for the first time, two are winner-take-all for the Republicans.

In Florida, the CBS News battleground tracker shows Donald Trump leading Senator Marco Rubio two-to-one in his home state -- so it could be "goodbye Rubio, Tuesday."

And it's do-or-die for Governor John Kasich in his home state of Ohio, where he and Trump are neck-and-neck.

Sarah Palin: Protesters engaged in "petty, punk-a** little thuggery"

Overshadowing all this: protests and occasional violence at Trump rallies -- and his role in it.

CBS News' Major Garrett reports on the Republicans

Kasich is in a tight battle with Trump in Ohio, the first contest the state's governor has a chance to win, reports Garrett.

"I'm not going to take the low road to the highest office in the land," Kasich said.

On Monday, he campaigned with 2012 GOP nominee Mitt Romney, who has urged Republicans to choose anyone but Trump.

"And so this is the guy Ohio needs to vote for. America is counting on you," Romney said.

Kasich told CBS News that Trump has gone too far.

Democrats accuse Donald Trump of inciting violence

"Well I think that you run a campaign where you divide one against other and making these incendiary comments at a rally that is a toxic environment," he said. "It is not healthy."

He added that the world is now watching.

"We are not in some third world country where they are slugging each other when they are trying to figure out who won an election, we are in America," he said.

Ted Cruz, fighting Trump for delegates in Tuesday's contests in Illinois and Missouri, faced an animal rights protester on Monday. Cruz defused the situation

"Isn't it amazing how we can have that conversation without anyone getting violent, anyone insulting anyone?" he said.

Frank Luntz: Anger surrounding Trump campaign getting worse

The stakes Tuesday are perhaps highest for Marco Rubio, trailing Trump badly in his home state of Florida. He also called for unity.

"Tomorrow we have a chance to make a powerful statement to the country and that is the Republican party is not going to be allowed to be hijacked by fake conservatives and people who go around dividing us against each other," he said.

But the Republican establishment has still not found a way to slow Trump's momentum.

House Speaker Paul Ryan, who has said he would work with Trump if he is the nominee, said in a radio interview Monday that protest tactics at Trump rallies were "unacceptable."

He also blamed Trump for some of the unrest.

"I think the candidates need to take responsibility for the environment at their events," he said. "There is never an excuse for condoning violence, or even a culture that presupposes it."

Paul Ryan: GOP nominee must reject bigotry

Ryan, like so many other Republican's wary of Trump, said anger among Republican voters is real, but warned against adding fuel to that fire. Ohio on Tuesday may well show how hot that fire burns and whether it can be cooled.

It could be a late night for the Democrats in Illinois on Tuesday -- CBS News' battleground tracker there shows Hillary Clinton and Bernie Sanders are neck and neck.

CBS News' Nancy Cordes is covering the Democrats

Sanders says the primary map is shifting in his favor. Tuesday he needs to prove it with strong performances in a trio of Midwestern states, reports Cordes.

In Ohio, Illinois and Missouri he's been hammering the message that delivered him a surprise win in nearby Michigan last week.

Can Bernie Sanders pull off another upset?

"The key difference between Secretary Clinton and myself is not only did I vote against every one of these disastrous trade agreements, I helped lead the opposition," Sanders said.

It's prompted Clinton to talk tougher on trade too.

"I will stop dead in its track any trade deal that hurts America," she said.

And argue, she is the candidate of the American worker.

She even tossed back a Guinness in blue collar Youngstown, Ohio, this weekend. And rallied with plumbers in Chicago this morning.

Clinton and Sanders set to do battle at the polls

"I'm going to fight for American labor," she said.

But it's been hard to ignore the elephant in the room: Donald Trump.

Especially since that elephant has been accusing Sanders of sending supporters to disrupt Trump rallies.

"Donald Trump is a pathological liar," Sanders said.

Both candidates have gotten increasingly caustic in their criticism of Trump. In Illinois, Clinton accused him of inciting mob violence, the kind that used to lead to lynchings.