Jeb Bush and Marco Rubio have maintained an awkward friendship for the duration of the presidential campaign. The two Florida Republicans have a shared political history and have always been mutually supportive. But things have changed.

Jeb is sliding in the polls and Rubio is ascendant. Once the clear establishment frontrunner, Jeb is now fighting for his political life. He's fallen out of a favor with GOP donors and is all but out of the race. Rubio, on the other hand, has been the primary beneficiary of Jeb's descent. Rubio is comfortably at third place in the national polls, trailing Trump and Cruz, and he's the most likely mainstream alternative to Trump and Cruz at this point.

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Little surprise, then, that Jeb has pivoted away from Trump and turned his guns on Rubio. Jeb's attacks on Trump haven't worked. Trump appeals to the base on a raw, visceral level. As a candidate, he's everything Jeb isn't: loud, confidently ignorant, and beyond reasoned criticism. Jeb won't win over any voters he's lost to Trump, and so focusing his attention there isn't productive.

But Rubio is a different story. Rubio's only path to the nomination is via the establishment lane of the party, and the same is true for Jeb. They're competing for the same constituency, and Rubio's rise made a confrontation between the two inevitable. And Jeb's new Rubio-centric strategy is a clear indication of where the Republican race is heading.

Jeb's super PAC, Right to Rise, just released an ad castigating Rubio for his absenteeism in the U.S. Senate. “Over the last three years,” the ad's narrator says, “Rubio has missed important national security hearings and missed more total votes than any other senator...Politics first: That's the Rubio way.”

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Bush has referenced Rubio's attendance record before (he even tried and failed to attack Rubio for this in a previous debate), but the new ad suggests a shift in tone, one Team Rubio has clearly noted. “Bush's team dishonestly omits that Marco is on the Senate's Intelligence Commitee, where he attended the highest-level briefings on the Paris attacks,” Rubio's campaign spokesman said. “No other candidate for president has received more classified Intelligence briefings or better understands the threats facing our nation today than Marco. It's sad to see Jeb's joyful campaign reduced to such intellectual dishonesty.”

Bush's super PAC will continue to run the anti-Rubio ads in Iowa in the coming weeks, hoping to slow Rubio's momentum there and elsewhere. It's unlikely to work, but Jeb is quickly running out of options. As it stands, he desperately needs to direct attention away from Trump, who has shown himself impervious to Jeb's attacks.

Rubio and Jeb are vying for the same votes, and so their rivalry will only escalate as the race moves forward. The same dynamic is playing out with Chris Christie, who is arguably the third most likely establishment candidate at this point. Realizing Trump is unimpeachable, Christie has followed Jeb's lead and focused his criticisms on Rubio. At a campaign event in Iowa this week, for instance, Christie dodged Trump-related questions and instead went after Rubio: “Dude, show up to work and vote no; and if you don't want to, then quit.”

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The establishment infighting will persist well beyond Iowa, as the non-outsiders turn increasingly on each other. New Hampshire will be especially competitive, as that's where Rubio, Bush, and Christie need a strong showing following a likely Cruz victory in Iowa. Whoever does the best in New Hampshire, then, will emerge as the leading establishment contender, and it appears the candidates all know it.