A super-PAC backing Marco RubioMarco Antonio RubioColbert: Students taking action on gun violence 'give me hope' Lawmakers feel pressure on guns Florida lawmaker's aide fired after claiming shooting survivors were 'actors' 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 GrassleyPavlich: The claim Trump let the mentally ill get guns is a lie Congress fails miserably: For Asian-Americans, immigration proposals are personal attacks Grassley, Dems step up battle over judicial nominees 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 ClintonWoman behind pro-Trump Facebook page denies being influenced by Russians Trump: CNN, MSNBC 'got scammed' into covering Russian-organized rally Pennsylvania Democrats set to win big with new district map MORE and Jeb Bush."

Rubio sits in third place in both national and Iowa polls behind GOP rivals Donald TrumpDonald John TrumpAccuser says Trump should be afraid of the truth Woman behind pro-Trump Facebook page denies being influenced by Russians Shulkin says he has White House approval to root out 'subversion' at VA MORE and Sen. Ted CruzRafael (Ted) Edward CruzOvernight Health Care: Trump eases rules on insurance outside ObamaCare | HHS office on religious rights gets 300 complaints in a month | GOP chair eyes opioid bill vote by Memorial Day HHS official put on leave amid probe into social media posts Trump, Pence to address CPAC this week MORE (R-Texas). But Bush lags even farther behind in the single digits.

The Iowa caucuses will be held on Feb. 1.