Marco Rubio picked up a flurry of endorsements Monday from key Republicans in his bid for the GOP presidential nomination, as the GOP establishment appeared to coalesce behind the Florida senator as its best hope to head off Donald Trump.

The endorsements, including support from former Senate Majority Leader and one-time presidential candidate Bob Dole, Sen. Orrin Hatch of Utah, Arkansas Gov. Asa Hutchinson and former Minnesota Gov. Tim Pawlenty, came after Jeb Bush exited the race and on the heels of a series of blunders by the campaign of Rubio rival Ted Cruz.

"I just feel that Rubio is the more serious candidate."

- Bob Dole

“What’s happened here is that a lot of mainstream Republicans realize if they want to stop Trump, their best bet is Marco Rubio, even over Ted Cruz,” GOP strategist Ford O’Connell told The Washington Times.

While it is not clear that the support of old guard Republicans will be enough to stop Trump in an election campaign dominated by insurgent candidates, party members expressed doubt that anger at Washington will be enough to defeat presumptive nominee Hillary Clinton.  

"I just feel that Rubio is the more serious candidate," Dole told Reuters. "And I feel he has the background to be able to really help turn this mess around.”

Rubio barely edged out Cruz in the most recent primary, in South Carolina, but finished well behind Trump, who establishment Republicans are wary of.

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Cruz, meanwhile, fired spokesman Rick Tyler on Monday over a video that falsely showed Rubio dismissing the Bible. Tyler had apologized a day earlier for retweeting a link to the "inaccurate story" involving a video purporting to show Rubio referring to the Bible and saying, "Not many answers in it."

The firing of Tyler followed other accusations of dirty tricks by the Cruz campaign, dating back to his victory in the Iowa caucuses.

Dole told ABC News his endorsement of Rubio was a direct result of Bush dropping out.

"Now that my good friend Jeb Bush is no longer running, I'm supporting Rubio," Dole told ABC's "Political Powerhouse" podcast.

Hutchinson, a respected former Congressman, likened Rubio to Ronald Reagan in a glowing statement of support.

"The more I've watched this election unfold, the more I've come to see that Marco Rubio is the only candidate who can unite our nation the way Ronald Reagan once did," Hutchinson said.

Arizona Sen. Jeff Flake said in a statement Monday that Rubio's message, generally less strident than the rhetoric of his rivals, is most likely to win the White House in November.

"I’m supporting Marco Rubio for president because he has the ideas, the principles, and the toughness necessary to bring reform to Washington," Flake said. "In an election where serious solutions are seriously lacking, Marco Rubio has proven that he can inspire more than just anger in Americans who are looking for an alternative to the status quo."