MIAMI: He has been attacked over countless issues in partisan Washington, but US President Barack Obama drew the line at this one: The idea that he is responsible for the rise of Donald Trump and the attendant Republican Party disarray.

“I have been blamed by Republicans for a lot of things, but being blamed for their primaries and who they’re selecting for their party (nominee) is novel,” Obama told a news conference.

“What I’m not going to do is to validate some notion that the Republican crack-up that’s been taking place is ... a consequence of actions that I’ve taken,” the Democratic president said.

Meanwhile, Ben Carson, retired neurosurgeon and onetime Republican presidential candidate, endorsed Donald Trump on Friday.

Carson had himself told Fox News radio that “I’m certainly leaning” towards a Trump endorsement, and an aide confirmed that Carson and Trump met recently.

On Thursday, Sen. Marco Rubio had criticized Trump for saying that Islam hates America, in a televised debate in Miami. Rubio, who faces a do-or-die contest in Florida on Tuesday, said that many Muslims were proud Americans.

“Presidents can’t just say whatever they want. It has consequences,” he said, to cheers from the audience.

The four Republicans heeded pleas from party leaders to have a civil debate.

Unlike in the last TV event, which was littered with personal insults, this one was more substantive with a focus on policy.

Floridians will vote on March 15 along with residents of Ohio and Illinois. All three big states are winner-take-all in the Republican delegate race, the first such contests in the 2016 cycle.

Many in the party see next Tuesday’s votes as the last best chance to derail the billionaire real estate mogul’s insurgent candidacy.

Rubio was championed by party luminaries as the best mainstream hope of derailing Trump, but he has performed dreadfully in several recent primary contests, including those on Tuesday.

With his campaign on the verge of fizzling, Rubio has stressed it is crucial to gang up on the frontrunner and has called on all Republicans to get behind him as the only candidate with a chance to beat him in Florida.

Rubio was far more polished and serious on stage Thursday than in recent weeks and debates, when his strategy appeared to be to deliver Trump a dose of his own belligerent medicine.