An already ugly presidential campaign has descended to a brutal new level. The question is no longer whether Donald Trump can be stopped on his march to the Republican presidential nomination, but whether it is possible to contain what he has unleashed across the country.

Violence at Trump's rallies has escalated sharply, and the reality-show quality of his campaign is taking a more ominous turn.

Yesterday, a man charged the stage in Dayton, Ohio, and Secret Service agents surrounded the Republican front-runner. Police named him as Thomas Dimassimo of Fairborn, Ohio.

The racially-tinged anger that has fuelled both his political rise, and stoked the opposition to it, has turned into a force unto itself. It has also brought a reckoning from his three remaining rivals for the Republican nomination, who are shedding their fear of provoking Trump and of alienating the raging slice of their party's base that has claimed him as its leader.


But Trump should not be viewed in isolation or as the product of a single election, President Barack Obama said in Dallas.

Obama said those who "feed suspicion about immigrants and Muslims and poor people, and people who aren't like 'us,' and say that the reason that America is in decline is because of 'those' people. That didn't just happen last week. That narrative has been promoted now for years."

A low point in the presidential campaign came on Saturday. Where Trump has delighted in mocking hecklers - and condoning attacks on them by his supporters - he was forced to cancel a rally at the last minute after protesters turned up by the thousands. That set off a chaotic scene in the arena at the University of Illinois at Chicago that left a handful injured and thousands agitated.

Trump's continued domination of the GOP race suggests there are no guardrails left in politics. Party elders and his opponents assumed that at some point, he would self-destruct.

But he has defied just about every norm. His candidacy and the sentiment it provokes have also stirred some disturbing historical comparisons. GOP political consultant Stuart Stevens, who was a top strategist for 2012 nominee Mitt Romney, said Trump's rhetoric is "almost verbatim" what segregationist George Wallace was saying in his third-party 1968 presidential campaign.

"I don't know what's in Trump's heart, but I don't care. What he's saying is really hateful," Stevens said. "What did the Democratic Party do with Wallace? They rejected him."

Some on the right accused the anti-Trump forces who shut down the rally in Chicago of being the true culprits, who denied the GOP front-runner an opportunity to exercise his constitutional right to free speech. "It's sad, No1, that you have protesters that resort to violence, that resort to threats of violence that resort to yelling and screaming and disruption to silence speech that they don't like," said Senator Ted Cruz, who is running a distant second to Trump in the primary. But his Republican opponents - all of whom have pledged to support Trump if he gets the nomination - said the New York billionaire cannot be held blameless.

"I think it is also true that in any campaign, responsibility begins and ends at the top," Cruz said.

"Look at the rhetoric of the front-runner in the presidential campaign," said Senator Marco Rubio. "This is a man who at rallies has told his supporters to basically beat up the people who are in the crowd and he'll pay their legal fees. Someone who's basically encouraged the people in the audience to rough up anyone who stands up and says something he doesn't like.

A man who jumped over a security barrier runs behind the dais where Republican presidential candidate Donald Trump is speaking during a campaign stop outside Dayton. Photo / AP
A man who jumped over a security barrier runs behind the dais where Republican presidential candidate Donald Trump is speaking during a campaign stop outside Dayton. Photo / AP

"I still at this moment continue to intend to support the Republican nominee," Rubio added, "but it's getting harder every day".

Ohio Governor John Kasich condemned Trump for creating a "toxic environment" that has led supporters and protesters to "come together in violence," but he too stopped short of saying he would not support the Republican rival if Trump secures the party's presidential nomination.

Their increasingly pointed criticism of Trump comes at a crucial moment in the GOP race, with primaries being held on Wednesday in five states that could either propel Trump to the nomination or give life to the effort to stop him. Most-closely watched will be Florida and Ohio, considered must-wins for home-state candidates Rubio and Kasich. And for the first time in this electoral season, delegates will be awarded on a winner-take-all basis, which means victories by Trump would accelerate his efforts to secure the nomination.

Trump has won Republican contests in 15 states, accumulating an estimated 458 Republican delegates of the 1237 he needs. Yesterday Rubio edged out Kasich in the District of Columbia's Republican primary which had 19 delegates at stake. Rubio had 37.30 per cent to Kasich's 35.54 per cent.

The decision to cancel the rally on Saturday was made by the Trump campaign, not the agencies charged with keeping him safe. After the man tried to breach the barricades around Trump yesterday, he was charged with disorderly conduct and inciting panic.

For months, Trump has been able to control - and even employ as foils - the hundreds of protesters who show up to his rallies to oppose what they consider divisive and racist.

Trump often says that he loves having protesters at his rallies, that they make his rallies fun. Plus, the interruptions are an opportunity to show him bossing and mocking liberals, often bellowing: "Get 'em out!"

In the past two weeks, however, these interruptions have increasingly eaten into Trump's speaking time and become more violent.

Florida, Ohio set to play crucial role

With Donald Trump's wins last week in Michigan and Mississippi, his chances of clinching the Republican nomination now depend heavily on what voters in Florida and Ohio do on Wednesday.

Both primaries are winner-take-all, with 99 delegates at stake in Florida and 66 in Ohio. So each could significantly boost Trump's 99-delegate lead over Ted Cruz, his nearest competitor.

If Trump sweeps Florida and Ohio, he could clinch the nomination by winning just more than half of the remaining bound delegates in play on Wednesday and afterwards, according to scenarios developed by Josh Putnam, a political science lecturer at the University of Georgia and blogger at Frontloading HQ.

So his opponents must win almost half to block him from amassing the 1237 delegates he needs.

It gets much harder for Trump to win - and significantly easier for his opponents to stop him - if Trump loses Florida, Ohio or both.

But he is the only candidate who appears likely to get the delegates needed before the Republican National Convention. Under all four possible Florida-Ohio scenarios, Cruz would need at least 77 per cent of delegates, which seems unlikely.

So are any of these scenarios within Trump's reach? Thus far he has won 34 per cent of votes and 43 per cent of all delegates - both shy of a majority. A Washington Post-ABC News poll last week pegged national Republican support for him at 34 per cent and Cruz at 25 per cent. Trump won Michigan with 37 per cent of votes, and Mississippi with 47 per cent.

While Trump hasn't won half the delegates, that's largely because, until Wednesday, states weren't allowed to award all of their delegates to the winner.

Starting this week, it will be much easier for Trump - or anyone else who can win states - to amass delegates very quickly.

- Washington Post, Bloomberg, AP