In Republican circles, one of the worse insults a candidate can hurl at another is to liken him or her to Barack Obama.

Former Florida Gov. Jeb Bush did that – and what is raising eyebrows is his target. Former mentee and possible current frenemy, Sen. Marco Rubio.

It was a new line in the sand for the Bush campaign, which until now has taken pains not to engage in open warfare with the junior senator from Florida as the two candidates vie to be the GOP nominee in the 2016 presidential race. Rubio, for his part, has seemed equally reluctant to take shots at Bush.

The new chapter in the Bush-Rubio rivalry is generating buzz. “Game on: Bush vs. Rubio,” was a recent headline in Politico, “Jeb’s Fight with Rubio Gets Personal” was the National Review’s, while the New York Times went with “Jeb Bush Questions Marco Rubio’s Leadership Acumen."

The spark for the turnaround, many political observers say, is Bush’s slipping ranks in GOP voter polls, along with Rubio’s rising momentum.

Rubio has been in fourth place recently in polls of GOP voters’ choice for president and those ahead of him are the so-called “outsiders” – real estate mogul Donald Trump, retired neurosurgeon Ben Carson and former Hewlett-Packard CEO Carly Fiorina.

On Thursday, when arguing that to vote on someone based on potential can be a dangerous leap of faith, Bush noted that he has the executive experience in political office to hit the ground running as president, according to the New York Times.

He said he wasn’t at all sure that Rubio can lead the nation from Day 1.

“It’s not known,” Bush said in an interview with MSNBC. “Barack Obama didn’t end up having [leadership skills], and he won an election on the belief that he could.”

It was the continued beating of a drum that started on Wednesday, in an interview with CNN in which Bush compared Rubio to Obama.

“Look, we had a president who came in and said the same kind of thing – new and improved, hope and change – and he didn’t have the leadership skills to fix things.”

That night, at a town hall in New Hampshire, Bush riffed about Rubio’s many missed Senate votes as he’s been campaigning for the Oval Office. “I think if you had a dock in pay strategy, you’d probably get more attendance,” he said.

Political experts say that it’s a clear sign that the former governor feels threatened by Rubio’s rising position.

“With Bush, they realize that Rubio is their problem, and if they don’t snuff him out, it’s really bad news,” Craig Robinson, a prominent GOP activist in Iowa, told Politico.

Not that Bush and Rubio have left each other completely untouched by unflattering comments during the early stage of the campaign.

Rubio, in particular, has made many veiled comments depicting Bush – without specifically naming him – as someone with old ideas from a bygone era. He has also said that senators make much better presidents than governors because they deal with national and international issues.

In his campaign launch, Rubio highlighted his youth and humble beginnings and contrasted them with the other candidates, who were born into privilege and who lack fresh ideas. (Bush had not yet announced, but was expected to.) It was also an obvious swipe at Democratic candidate Hillary Clinton, but, many noted, it applied equally well to Bush.

Donald Trump, who had not been paying much public attention to Rubio, has trained his guns on him.

“You look at what happened to Scott Walker, and Rubio’s strategy looks smarter and smarter every day: bide your time, be patient," Fergus Cullen, a former New Hampshire GOP chairman, told Politico. "You've got to wait for the game to come to you. But there's a sense of urgency for other candidates concerned about Rubio, which is why they're taking the game to him and trying to draw him out."

“If Rubio is ever going to emerge,” said Robinsona, "it’s going to be now, and I think the rest of field recognizes that.”

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