A super-PAC backing Marco RubioMarco Antonio RubioOvernight Defense: Tillerson, Trump deny report of rift | Tillerson says he never considered resigning | Trump expresses 'total confidence' in secretary | Rubio asks Army to kick out West Point grad Rubio asks Army to kick out West Point grad with pro-communist posts GOP establishment doubts Bannon’s primary powers MORE is hitting back at Jeb Bush, calling him "desperate" for his attacks on the Florida lawmaker's Senate attendance.

“Desperate candidates say desperate things, and Jeb Bush is desperate,” the narrator says in the new TV ad titled "Security," which launched Thursday from Conservative Solutions PAC. The ad will air statewide in Iowa.

It comes after an ad from Right to Rise, a super-PAC supporting Bush, on Tuesday, knocked Rubio for missing a top-secret briefing days after the Paris terrorist attack to fundraise.

Rubio campaign’s has called the charge unfounded, noting that he attended a select committee hearing regarding the attack days before the briefing.

Bush and other GOP rivals including New Jersey Gov. Chris Christie have chided the Florida senator for missing votes and Senate hearings.

The pro-Rubio spot also invokes Sen. Chuck GrassleyCharles (Chuck) Ernest GrassleyRepublicans jockey for position on immigration House clears bill to combat crimes against elderly Grassley: DACA deal wouldn't need border wall funding MORE (R-Iowa), displaying a clip from a local TV station with a headline saying that the state’s popular senator is defending Rubio.

“His attacks on Marco Rubio have been dismissed and debunked by our own Sen. Grassley,” the ad says. “The fact- Rubio has attended more classified national security briefings this year than any other candidate.”

The ad closes by describing Rubio as "the conservative most feared by Hillary ClintonHillary Diane Rodham ClintonChris Murphy’s profile rises with gun tragedies DNC, RNC step up cyber protections Gun proposal picks up GOP support MORE and Jeb Bush."

Rubio sits in third place in both national and Iowa polls behind GOP rivals Donald TrumpDonald John TrumpIvanka Trump pens op-ed on kindergartners learning tech Bharara, Yates tamp down expectations Mueller will bring criminal charges Overnight Cybersecurity: Equifax security employee left after breach | Lawmakers float bill to reform warrantless surveillance | Intel leaders keeping collusion probe open MORE and Sen. Ted CruzRafael (Ted) Edward CruzWhatever you think the Alabama special election means, you’re probably wrong This week: Congress gets ball rolling on tax reform Week ahead: Senators work toward deal to fix ObamaCare markets MORE (R-Texas). But Bush lags even farther behind in the single digits.

The Iowa caucuses will be held on Feb. 1.