A nasty battle has broken out in the Republican presidential field, and it doesn’t involve Donald Trump.

As the calendar draws closer to the Iowa and New Hampshire contests, the second tier of GOP candidates – along with the super PACs supporting them – are unloading on each other in a blitz of ads, videos, tweets, stump speeches and interviews. The acrimony is at a level until now unseen, in a race dominated by vitriolic squabbles between Trump and whichever candidate of the moment displeases him.

Now, with Trump training his focus on Democratic front-runner Hillary Clinton, the rest of the pack is fighting to rise above. The latest round involves Florida Sen. Marco Rubio, New Jersey Gov. Chris Christie, Ohio Gov. John Kasich and former Florida Gov. Jeb Bush.

The super PAC backing Bush is out with a new ad blasting Rubio for missing a Senate meeting after the Paris terror attacks – and another contrasting Bush’s gubernatorial record against those of Christie and Kasich.

“Politics first, that’s the Rubio way,” the first ad says, slamming Rubio for fundraising while missing meetings and hearings on the Hill.

Rubio has long battled criticism of his attendance record in the Senate. In 2015, he has missed about 35 percent of roll call votes, according to GovTrack.us. That's more than any of the other senators running for president.

The latest headlines on the 2016 elections from the biggest name in politics. See Latest Coverage →

“Dude, show up to work,” Christie told a crowd in Iowa Tuesday, ribbing Rubio for missing a spending bill vote.

Rubio, speaking with Fox News, defended his missed votes on Wednesday. He said Washington is “completely broken” and “more than half the things that happen in Washington are just for show or for talk.”

As for Christie, he said, “He’s never in New Jersey. He’s gone half the time.”

On Tuesday, Rubio also fired back against the pro-Bush ad, charging that Bush is getting “increasingly negative in his attacks.”

Right to Rise USA, the pro-Jeb Bush super PAC, is spending $1.4 million on the ad buy which begins airing this week in the Hawkeye State.

While these fights are playing out in Iowa, many of these candidates are fighting even harder for New Hampshire.

Right now, Trump and Texas Sen. Ted Cruz are jockeying for the lead in Iowa, with the rest of the field far behind. But while Trump also leads in New Hampshire, five candidates – Rubio, Christie, Cruz, Kasich and Bush – are tangled up in a close race below him in that state. Candidates like Christie and Kasich especially, who have struggled to gain traction elsewhere, are banking on a breakout performance in New Hampshire to gain momentum in the race.

This could explain why Kasich’s campaign and its aligned super PAC are firing back hard at Bush.

In response to the latest ad, Kasich press secretary Rob Nichols said: “The latest ad from Jeb’s team forgot to check the box for ‘Which governor is living in the past because he has no new ideas for fixing anything?’ You only attack those you fear and who’s beating you, so this latest attack by Jeb on Gov. Kasich only reaffirms the governor’s strength in New Hampshire. It’s actually flattering.”

The Kasich campaign also put out a cheeky video casting Bush as out of touch with the times.

“Jeb loves the good ole’ days,” the video declares, before showing vintage footage of things like Sony’s Betamax and the “Macarena,” the 1994 hit by Los Del Rio.

Fox News’ Serafin Gomez and Jessica O'Hara and The Associated Press contributed to this report.