Sen. Marco Rubio is not leading in the early-nominating states of Iowa, New Hampshire or South Carolina, yet he’s become the target of attacks by rivals.

With Jeb Bush and Chris Christie assailing his missed votes in Washington, and Ted Cruz hammering his past support of comprehensive immigration reform, Rubio, the Florida senator, has been on defense – a lot.

On Tuesday, his campaign raced to quell attention on a new ad airing in Iowa from a super PAC supporting Bush. The 30-second spot attacks Rubio for missing briefings in Washington following the recent terrorist attacks in Paris and San Bernardino, and says he places politics – he attended fundraisers in California and Louisiana during those meetings – over national security. Christie, who himself has taken flak for missing time away from his job as New Jersey governor to campaign for president, even hit Rubio for a missed vote on Congress’ big end-of-the-year spending bill.

"Dude, show up to work and vote no,” Christie said at a town hall in New Hampshire on Tuesday. “Just show up to work and vote no. And like if you don't want to, then quit."

Bush, Christie and Rubio are competing to win New Hampshire. Caucus-goers in Iowa are set to kick off the 2016 election on Feb. 1, and Rubio is in third place there, behind Cruz and billionaire businessman Donald Trump, based on an average of several surveys. For Bush and Christie, sullying Rubio’s Iowa showing could help them in the Granite State, which will hold its primary eight days later.

Rubio’s time away from the Senate has been used against him for much of the fall. He has missed the most time from his day job out of the three senators running for the GOP presidential nomination, according to an analysis by C-Span.

His campaign fired back against the Bush attack ad, calling it baseless and enlisting several fellow lawmakers to defend Rubio. Rubio’s campaign has also said previously that though he missed the briefing after the Paris attacks, he had attended a classified session for senators on the intelligence committee the night before.

“I’m proud to support Marco, who understands more than any other candidate the threats facing our country, knows how we can defeat ISIS, and is ready to be commander-in-chief,” Rep. Chris Stewart (R-Utah) said in a statement released by Rubio’s campaign.

“Jeb Bush is better than the Los Angeles-based super PAC trying to deceive voters,” Stewart added, a dig at the pro-Bush super PAC run by L.A.-based operative Mike Murphy.

Rubio is no stranger to California, either. Out of 116 trips this election cycle, he’s traveled to the Golden State nine times, which outpaces Bush’s trips to the state, according to a National Journal tracker of 2016 candidates’ travel.