He says he’s frustrated with the job, and he hardly bothers to show up to vote.

So why doesn’t Florida Sen. Marco Rubio heed his critics — including his one-time mentor, former Florida governor Jeb Bush — and leave the Senate so he can pursue the presidency full time?

Rubio’s campaign won’t give a direct response, referring only to the senator’s previous statements.

Rubio has said that he shows up for votes when they matter and that his office continues to help constituents and provide other services. He notes that plenty of other senators who ran for the White House also had high absentee rates, and none quit the Senate.

“A lot of these votes won’t mean anything,” he told CNN. “They’re not going to pass. And even if they if they did, the president would veto it.”

The short answer on why Rubio won’t resign is that it probably wouldn’t help his campaign to be labeled a quitter. And his Senate seat gives Rubio a high-profile platform to weigh in on key issues.

“Marco can hold that as an ace in the hole,”said Keith Fitzgerald, a political science professor at New College of Florida in Sarasota and a former Democratic state lawmaker who served with Rubio in the Florida House. “At some point, he can use his position to gain center stage by being a key opponent to something that they want to do or possibly an advocate of something they want to accomplish. So why give that up?”

Rubio and his defenders say he’s really no different from plenty of other senators who have run for the Oval Office.

They point to John McCain of Arizona, John Kerry of Massachusetts and Barack Obama of Illinois, all of whom sacrificed substantial portions of their day jobs to run for president. This year, Sen. Ted Cruz of Texas and Sen. Lindsey Graham of South Carolina also have missed a large number of votes while campaigning for the presidency.

From 1972 through the 2016 election cycle, 45 sitting senators have sought the presidency but only one – Bob Dole of Kansas – resigned before the election, according to Smart Politics, a nonpartisan blog run by University of Minnesota Professor Eric J. Ostermeier.

But Rubio’s missed votes have become a prominent talking point for his critics. That includes Bush, who was close to Rubio when Bush was Florida’s governor and Rubio was ascending to speaker of the Florida House.

“Marco, when you signed up for this, this was a six-year term, and you should be showing up to work,” Bush told Rubio at Wednesday’s GOP presidential debate.

It’s not surprising that Bush, struggling in the polls, would go after his former protege on absenteeism. Rubio has missed more floor votes than any other senator this year (including all eight votes this week). And Bush and Rubio are competing to be the No. 1 GOP establishment candidate, with Rubio in the lead.

“I don’t remember you ever complaining about John McCain’s vote record,” Rubio shot back. “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.”