Republican senators are coming around to the view that Jeb Bush is unlikely to win the party’s nomination for president and that freshman Sen. Marco Rubio Marco Antonio RubioOvernight Defense: Pentagon redirects pandemic funding to defense contractors | US planning for full Afghanistan withdrawal by May | Anti-Trump GOP group puts ads in military papers Democrats step up hardball tactics as Supreme Court fight heats up Press: Notorious RBG vs Notorious GOP MORE (Fla.) is the most viable prospect for the general election.

Rubio has had plenty of support among Beltway pundits since the outset of his campaign but Bush’s poor performance in the last Republican debate, together with his declining poll numbers, have begun to shift sentiment in even the upper echelons of the GOP’s establishment.

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“Marco’s in the driver’s seat. There’s a lot of disappointment in Bush’s performance,” said one Republican senator, who requested anonymity to discuss the race candidly.

The lawmaker, however, left the door ajar to the possibility of a Bush comeback, noting that “expectations for Jeb are so low that it won’t be hard to exceed them.

“He just needs a little momentum,” the lawmaker added.

Most GOP senators are waiting for the race to shake out before venturing to make a public endorsement, in case there’s a late reversal of fortune, which has happened in previous election cycles.

But one pro-Rubio senator said “it’s nearly unanimous” in the Republican conference that Bush is floundering and Rubio is on the rise.

“You can’t look at the overall picture and not think it,” said the senator. “The polling shows it. It’s just amazing what’s happened.”

Rubio picked up three Senate endorsements last week — Sens. Cory Gardner (R-Colo.), Steve Daines (R-Mont.) and Jim Risch (R-Idaho) — matching Bush, who so far has thee: Sens. Orrin Hatch (R-Utah), Dean Heller (R-Nev.) and Susan Collins (R-Maine).

Polls show Rubio surging and Bush dropping since the last debate in Boulder, Colo., on Oct. 28.

A nationwide Quinnipiac University Poll released Wednesday showed Rubio jumping 5 points to 14 percent, good enough for third place behind Donald Trump and Ben Carson.

“His stock is up,” said a senior Republican senator of Rubio. The senator also expressed sympathy for Bush saying that the former Florida governor’s decline “must be a humbling experience.”

Bush has dropped 6 percentage points since September in the Quinnipiac poll, now registering only 4 percent support. He shows the worst net favorability rating of any candidate, with 25 percent of Republican voters saying they view him positively and 58 percent viewing him negatively.

“Obviously those are not good stats,” said one Republican senator, who is pledging to stay neutral.

A Senate Republican who backed Bush’s father, former President George H.W. Bush, in a presidential primary, said Jeb “is in big trouble.”

“He’s cross-ways with the Republican base. They see him as the epitome of a bought and paid-for establishment candidate,” the lawmaker said. “I don’t see how he can get back in there.”

The source said even if he “showed more energy,” Bush would have trouble explaining his past statements in favor of a path to citizenship for illegal immigrants and Common Core education standards.

Bush has since said he does not support a path to citizenship but instead a path to earned legal status.

One Bush ally said the former Florida governor was “probably” finished after he awkwardly tried to attack Rubio in the Colorado debate only to be brushed aside.

Bush’s ineffectual jab was the buzz of the weekly Thursday Republican lunch.

“A bunch of us were talking about it,” said the lawmaker, who lamented the timing of Bush’s attack on Rubio for missing votes. “He stepped in right after Marco had a big applause line.”

Rubio got roaring applause at the debate when he accused the mainstream media of a double standard after the Sun Sentinel called for his resignation, even though it endorsed former Sen. John Kerry (D-Mass.) in 2004, who also missed Senate votes to campaign for president.

Bush awkwardly followed up Rubio’s zinger by admonishing him to show up for work or otherwise resign, taking the Sentinel’s side.

Other Republicans dismiss the chatter that Bush is in a tailspin, citing Sen. John McCain John Sidney McCainCindy McCain endorses Biden: He's only candidate 'who stands up for our values' Biden says Cindy McCain will endorse him Biden's six best bets in 2016 Trump states MORE’s (R-Ariz.) comeback in the 2008 Republican presidential primary, when he captured New Hampshire after his campaign had seemed on the brink of collapse.

“McCain was way, way down before New Hampshire,” said a Republican senator, who went on to excoriate the rollicking, wide-open nature of this year’s primary.

“I wonder how George Washington and Abraham Lincoln would do in this format. They would probably be way, way down,” he said.

McCain noted that recent polls show a substantial percentage of New Hampshire voters have yet to make up their mind.

“They pride themselves in not making up their minds. You know the old joke of Mo Udall’s, a guy in Concord is asked, ‘What do you think about Mo Udall for president?’ and he says, ‘I don’t know, I’ve only met him twice,’” McCain said of the former House lawmaker who ran for president in 1976.

Heller, whose home state of Nevada hosts an early contest in the primary, praised Bush earlier this year as a “proven leader.”

“The nation needs the pragmatic, conservative, principled leadership that only Jeb provides. And Jeb understands why Nevada is important when appealing to the majority of American voters,” Heller said in a statement issued in August when he endorsed him.

Another Republican senator argued that Bush still has a better chance than Sen. Ted Cruz (R-Texas), whom he described as too far to the right.

“I think it comes down to Bush, Marco or [Ohio Gov. John] Kasich,” the senator said, dismissing Cruz has having “a pretty low ceiling.”

Most Republican senators don’t think Trump or Carson, the frontrunners in national polls, are likely to win the nomination because of their lack of experience. Neither man has ever held any elected office

--This article was updated at 1:27 p.m.

An earlier version of this article did not mention that Sen. Susan Collins (R-Maine) has endorsed Bush.