Mitt Romney launched a new Web video attacking the conservative credentials of Newt Gingrich, who is emerging as his main rival for the GOP presidential nomination.

The ad, out Friday, hits Gingrich for comments he made about House Budget Committee Chairman Paul RyanPaul RyanThe Hill Interview: Budget Chair Black sticks around for now Gun proposal picks up GOP support GOP lawmaker Tim Murphy to retire at end of term MORE's (R-Wis.) budget plan.

It splices together news reports and clips of political pundits reporting on comments Gingrich made on Ryan's plan, which would turn Medicare into a voucher-like system for those under 55. In an interview Gingrich did shortly after he jumped into the presidential race, the former House Speaker criticized the plan, calling it "radical."

"I don't think right-wing social engineering is any more desirable than left-wing social engineering," Gingrich said on "Meet the Press."

Gingrich's comments received strong criticism from the right as Ryan's deficit-reduction plan was increasingly being seen as a conservative litmus test. Ryan's plan passed the House but failed in the Senate.

The ad comes as recent polling shows Gingrich either neck and neck with Romney or ahead of the former Massachusetts governor. It also appears as conservative voters — particularly in the early-caucus state of Iowa — are still searching for a Romney alternative. A Fox News poll out Thursday showed more voters thought of Gingrich as more conservative than Romney, 53 percent to 40.

The Web ad ends with a slide of Gingrich with the words "With friends like Newt, who needs the left?"

Watch the ad below: