With less than five weeks before voters begin weighing in on the 2016 GOP presidential nomination, the establishment contenders — who until now have been relatively restrained — have begun aiming their fire at each other.

The tactical shift on the part of the candidates and their allies reflects a long-standing assumption as to how this crowded nomination battle is likely to play out.

Many believe the race will come down to a one-on-one contest between an “outsider” who channels the angry Republican base and a candidate more in line with the wishes of the party hierarchy. The establishment pick has almost always prevailed in the past, though it is far from certain that will be the case in 2016.

The Sunday after a contentious Republican debate, presidential candidates Jeb Bush, Marco Rubio, Ben Carson and Rand Paul went after rival Donald Trump in television appearances. They attacked Trump's relationship with Putin, his proposed ban on allowing Muslims to enter the country and his seriousness as a candidate. (Sarah Parnass/The Washington Post)

The insurgent faction of the party appears likely to rally around either front-runner Donald Trump or the ascendant Sen. Ted Cruz (R-Tex.). The leading possibilities on the establishment side include three sitting or former governors and a Florida senator — all of whom are running far behind Trump. But before any of them can get a shot at taking him on, they must deal with one another.

Sen. Marco Rubio (R-Fla.) on Tuesday found his record of Senate absences under attack from two directions: a blistering ad in Iowa by a super PAC supporting former Florida governor Jeb Bush and a taunt by New Jersey Gov. Chris Christie.

The super PAC supporting Republican presidential candidate Jeb Bush released an ad attacking Sen. Marco Rubio's voting record. (Youtube/Right to Rise USA)

“Dude, show up to work,” Christie said during a town-hall meeting in Muscatine, Iowa.

Rubio was making his own swing through Iowa, which is set to hold the first contest of the primary season on Feb. 1. He responded at a news conference hours later in the town of Clinton.

“You know, Chris has been missing in New Jersey for half the time,” Rubio said. “But candidates, I think, as we get down the stretch here, some of them get a little desperate and a little nasty in their attacks, and that’s fine.”

He dismissed the attack ad by the pro-Bush super PAC Right to Rise USA as “par for the course. I mean obviously you’ve seen that as we get closer to Election Day and millions of dollars of spending have not changed [Bush’s] fortunes, he’s become increasingly negative in his attacks.”

Meanwhile, Right to Rise USA also launched a spot in New Hampshire contending that the gubernatorial records of Christie and Ohio Gov. John Kasich do not stack up against that of Bush. The ad was gentler in tone than the super PAC’s assault on Rubio, but it attempted to draw a contrast on the experience that all three of the candidates consider to be their greatest asset.

New Day in America, a super PAC supporting Kasich, responded: “What Team Jeb has failed to address is the political baggage dragging behind Bush and Christie. The country doesn’t have an appetite for another Bush, or another Clinton, for that matter. As for Governor Christie, his mishandling of his state budget and the ‘Bridgegate’ scandal have earned him a 60 percent unfavorable rating from those who know him best — the people of New Jersey.”

The crossfire, said GOP political consultant Alex Castellanos, is beginning to look like “a ‘Fistful of Dollars’ gunfight,” referring to the 1964 spaghetti western that launched Clint Eastwood to stardom.

“It’s now down to the last five weeks here,” Christie told reporters in Muscatine. “We need to make distinctions between candidates, not just on issues but on experience, so I’ll be talking about it. I think it’s an important distinction to make.”

The young and charismatic Rubio is regarded by many as potentially the strongest contender running in the establishment lane — if he can find an opening.

“It’s pretty clear in this race that Marco Rubio is the candidate of the future. If he ever breaks through the line, he’s got a lot of running room,” said Castellanos, who is not working for any of the 2016 candidates. “The only one who everyone fears for his explosive potential is Rubio.”

Right to Rise USA said it plans to spend $1.4 million on the new ad in Iowa, in which an announcer says: “Over the last three years, Rubio has missed important national security hearings and missed more total votes than any other senator. Politics first — that’s the Rubio way.”

Christie sounded a similar theme during a town hall in Muscatine, where he noted that Rubio had announced his opposition to a $1.8 trillion, year-end spending bill and tax package, but then failed to show up to vote against it. It passed the Senate 65 to 33.

“Just show up to work and vote no,” Christie said. “And if you don’t want to, then quit.”

Rubio spokesman Alex Conant said his campaign is unfazed, noting that Rubio’s skill at dealing with barbs during the candidates’ debates has only increased his stature.

“Obviously, we feel very good about our campaign, and where we’re at. With success comes scrutiny,” Conant said. “We’ve been taking some incoming for several months now, and we’re not worse for the wear.”

Rubio does have the worst attendance record of all the senators, including the five who have run for president this year — a fact he does not dispute. A C-SPAN analysis released last week found that Rubio cast votes for just 219 of the 339 recorded Senate roll-call votes in 2015 — a 65 percent attendance record. He spoke on the Senate floor eight times — just 5 percent of the days that the Senate was in session, C-SPAN said.

“I’m running for president, because I want to change the direction of this country, and it will require me, for the time being, to miss some votes in the U.S. Senate, because I want those votes to matter again,” Rubio said.

Before Tuesday’s exchanges, the most high-profile attack on Rubio’s attendance record had been launched by Bush during an October Republican presidential debate. Rubio shot back with a response that threw Bush off balance.

“Someone has convinced you that attacking me is going to help you,” Rubio told Bush, and then recounted how other senators who ran for president had equally bad attendance records that had not drawn much comment.

Sen. Marco Rubio (R-Fla.) came out swinging during the CNBC debate and had some big applause lines. See his most memorable moments of the third Republican debate. (Jhaan Elker/The Washington Post)

In his own appearances recently, Bush has been focusing most of his attacks on Trump and Hillary Clinton.

By law, candidates are not allowed to coordinate with the super PACs that support them; however, Right to Rise USA is run by political consultant Mike Murphy, who has been one of Bush’s closest political advisers for most of his political career.

O’Keefe reported from Nashua, N.H. Rucker reported from Muscatine, Iowa.