BOULDER, Colorado — Jeb Bush's beleaguered presidential campaign was dealt another blow at Wednesday's Republican primary debate when the candidate's high-stakes gambit to attack an opponent he once considered his protégé badly backfired.

Bush entered the CNBC-hosted debate with stubbornly lackluster poll numbers, unimpressive fundraising, and some of the worst press he has received all year — fueled by morbid anonymous quotes from skittish donors and allies. As one Bush fundraiser memorably told the Washington Post last week, "It feels very much like a death spiral, and it breaks my heart."

Bush looked to escape the "death spiral" — and revive his supporters' confidence in his 2016 viability — with a commanding debate performance Wednesday, which included lambasting Marco Rubio for his record of skipping Senate votes.

"I'm a constituent of the senator and I helped him and I expected that he would do constituent service, which means that he shows up to work," Bush said, describing the young Floridian, whose political rise he has long supported, as a "gifted politician."

Bush then angled toward Rubio, who was standing next to him onstage, and addressed him directly by his first name.

"But Marco, when you signed up for this, this was a six-year term, and you should be showing up to work," he said. "I mean, literally, the Senate — what is it, like a French workweek? You get, like, three days where you have to show up? You can campaign, or just resign and let someone else take the job."

"I get to respond, right?" Rubio asked the moderators, coolly. He then pointed out that the 2008 Republican nominee, Sen. John McCain, missed Senate votes during his presidential campaign, as well.

"I don't remember you ever complaining about John McCain's vote record," Rubio told Bush. "The only reason why you're doing it now is because we're running for the same position, and someone has convinced you that attacking me is going to help you."

The loud applause from the audience that followed drowned out Bush's bid at formulating a comeback — thus bringing an end to the candidate's attempt at a game-changing moment.

Bush's attack on Rubio was clearly premeditated, a fact evidenced by his campaign's quick-draw email blast to reporters highlighting the "French workweek" line minutes after he said it. But according to Florida-based GOP strategist Rick Wilson, Bush blew the moment by giving up the element of surprise in the days leading up to the debate.

"Never mark your ambush with flashing lights and traffic cones. Everyone in Florida politics heard this attack was coming since the weekend, and Marco was absolutely ready for it," said Wilson, who actually hinted at the coming dig on Twitter before the debate began. "This was not Jeb at his best."

In the post-debate spin room, Bush's campaign manager, Danny Diaz, acknowledged Rubio's rhetorical skill, but used it to draw comparisons to Barack Obama circa 2008.

"No one is going to argue that Sen. Rubio isn't an outstanding performer," said Diaz. "But there's a difference between an outstanding performer and one that has delivered over and over again. We have an outstanding performer who's been in the White House for seven years, and I think most Americans and largely all Republicans are dissatisfied with the outcome."

Bush spent the rest of the debate turning in the same sort of awkward, listless performance that has marked his other onstage appearances this year. He spoke for just six minutes and 39 seconds in total, getting less airtime than every other candidate on the stage except for Rand Paul, according to NPR Politics.

When a moderator asked him whether he would support a hypothetical budget deal that included $10 of spending cuts for every $1 of tax increases, he struggled to articulate an answer, and ended up fumbling for a joke.

"You find a Democrat that's for cutting taxes — cutting spending $10, I'll give them a warm kiss," he said.

If Bush's aim was to calm his supporters' nerves and dispatch Rubio, who has passed him in some recent national polls, Wednesday's debate was likely counterproductive. Before the event was even over, establishment figures in the GOP were panning his performance.

"Simply put, he whiffed," said one South Carolina Republican, who is neutral in the 2016 race. "He needed a moment to assuage donor fears and it backfired. As much as people may say the Bush name is a hindrance, the reality is that his last name is the only thing keeping him in the conversation right now."

But even Bush's famous last name may not be enough to keep his supporters in line now.

Asked after the debate how many phone calls from Bush donors the Rubio campaign had already fielded, Rubio's spokesman, Alex Conant, told BuzzFeed News coyly, "I don't have a number, but we've gotten calls."