Jeb Bush’s comeback tour has yet to convince at least one key group of his U.S. presidential potential: Republican senators. Increasingly, GOP senators have shifted their support of the two Floridians to their colleague Marco Rubio from the former state governor.

“Marco’s in the driver’s seat. There’s a lot of disappointment in Bush’s performance,” an unnamed Republican senator told the Hill, one of several who described campaign momentum swinging toward Rubio and away from Bush.

Three GOP senators endorsed Rubio last week, while Bush to date has racked up only two endorsements in the Senate.

Faced with sinking poll numbers and widely criticized debate performances, Bush and his campaign have attempted to engineer a turnaround tour, positioning the candidate as the most serious and well-informed on the extensive Republican slate. But he has struggled to shore up his conservative bona fides among GOP voters who see him as a compromised member of the establishment.

Meanwhile, Rubio has enjoyed a surge in opinion polls the past few weeks, owing in part to a commanding debate performance in late October. Several senators suggested to the Hill that Bush’s ill-timed attack on Rubio during that debate precipitated the reversal in the two candidates’ fortunes. “A bunch of us were talking about it,” one lawmaker said of the debate’s reception in among Republican legislators. “He stepped in right after Marco had a big applause line.”

It’s not just supporters on Capitol Hill who have defected to Rubio. Billionaire hedge-fund manager Paul Singer threw his substantial weight behind Rubio, while suggesting other campaign contributors do the same.

Since the debate, third-place Rubio’s support has risen to 12 percent from 9 percent in an average of national polls maintained by RealClearPolitics. Bush, who has seen his support slide almost uninterruptedly since July, has fallen to less than 6 percent, which has him in fifth place behind the front-runners, businessman Donald Trump and retired neurosurgeon Ben Carson, Rubio and U.S. Sen. Ted Cruz of Texas.