A super-PAC backing Marco RubioMarco RubioOvernight Defense: Commander calls North Korea crisis 'worst' he's seen | Trump signs VA order | Dems push Trump to fill national security posts What’s with Trump’s spelling mistakes? Boeing must be stopped from doing business with Iran 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 GrassleyChuck GrassleyComey to testify before Senate Judiciary Committee GOP to kill language exempting staff from new ObamaCare repeal bill House cyber chairman wants to bolster workforce 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 Rodham ClintonSamantha Bee roasts Trump at mock correspondents' dinner Dems seeing big increase in midterm House candidates When it comes to Israel, Trump’s first 100 days were one big fail MORE and Jeb Bush."

Rubio sits in third place in both national and Iowa polls behind GOP rivals Donald TrumpDonald TrumpMcMaster to South Korea: US will pay for missile defense system Comedian Hasan Minhaj blasts Trump, media at WHCA dinner White House correspondents' chief: 'We are not fake news' MORE and Sen. Ted CruzTed CruzTrump in campaign mode at NRA convention Trump’s hands are tied on 9th Circuit Schumer: Trump's handling of North Korea 'all wrong' MORE (R-Texas). But Bush lags even farther behind in the single digits.

The Iowa caucuses will be held on Feb. 1.