The ad calls Sen. Marco Rubio a flip-flopper, an amnesty fanatic, and someone who lacks the stature – literally – to be president of the United States.

The shots at the presidential candidate are coming from the super PAC supporting the campaign of his one-time friend and mentor, former Florida Gov. Jeb Bush, who is barely registering in many polls across the country after having entered the presidential race as the presumptive GOP nominee.

And some Republicans don't like it one bit.

Many GOP insiders fear that the jabs – by the super PAC Right to Rise – which they view as cheap shots that are also misleading and will do more than harm than good, according to Politico.

They see Bush, who has been languishing at about 5-6 percent in opinion polls, as having little chance of becoming the establishment GOP alternative to conservative firebrands Donald Trump and Sen. Ted Cruz. Rubio, by contrast, has maintained a formidable position in many national, regional and state polls, and has delivered strong performances in debates.

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Establishment GOP insiders see Rubio as their most potent antidote to Cruz and Trump, and fear that the Bush super PAC’s growing efforts to undermine the Florida lawmaker will end up helping Cruz and Trump.

“This is something Jeb Bush has to decide. Does he want his legacy to be that he elected Donald Trump or Ted Cruz?” Politico quoted Stuart Stevens, the GOP strategist who ran Mitt Romney’s 2012 campaign, as saying. “He can’t control that super PAC, but he ought to call on the super PAC to stop and stop attacking people with whom he mostly agrees.”

Bush himself backed off hitting hard at Rubio, including in GOP debates, after it seemed only to bolster Rubio and cast Bush in a negative light. Republicans say that having the super PAC continue the approach to criticizing Rubio probably won't produce better results for Bush.

“This thing has been mismanaged and screwed up since the beginning,” Politico quoted an unidentified Jeb Bush backer who also worked in both previous Bush administrations. “It’s gotten to the point where the old-timers are saying ‘it’s really sad.’ How as presumptive leader with $100 million in the bank did you get yourself in a box where you have to attack Rubio and [New Jersey Gov. Chris] Christie to win your lane?”

In its recently-released third anti-Rubio advertisement, Right to Rise cast the Florida senator as a chameleon on the issue of immigration.

It shows Rubio as a weather vane on a roof, pointing one way then the other, to depict what it characterizes as his changing positions on immigration.

“Marco Rubio ran for Senate saying he opposed amnesty,” a narrator says. “Then he flipped, and worked with liberal Chuck Schumer to co-author the path to citizenship.”

“He threatened to vote against it, and then voted for it. He supported his own Dream Act, and then he abandoned it.”

Some experts say that, if anything, immigration is not the issue to use to hammer away at Rubio, since Bush himself has expressed support in the past for giving undocumented immigrants a path to legal status.

Bush defenders say he’s still a player in the race and that it would make no sense for him to go after Trump and Cruz, two candidates whose supporters he has little chance of getting to convert to his side.

Christie, they say, has hurled direct, insulting comments at Rubio’s camp, and isn’t getting the kind of frowns in response from establishment Republicans.

“I’ve not heard Jeb say Rubio is trying to slime his way to the White House,” said Ron Kaufman, a Bush backer and long-time GOP operative in Massachusetts, to Politico. “Why should Jeb Bush, the only candidate in the Republican race with a legitimate national network, why is it not okay for him to draw contrasts when all of these guys are battling to be the finalist in the centrist conservative lane?”

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