Newt Gingrich has a message for John McCain: Welcome to the 2012 race.

On Wednesday, a super PAC backing Gingrich welcomed McCain’s endorsement of former Massachusetts governor Mitt Romney (R) by seizing on a Web ad, “Two Mitts,” that the Arizona Republican had run against Romney during the 2008 GOP nominating battle.

The spot juxtaposes clips of Mitt Romney stating contrasting positions on issues including abortion rights and gun control throughout his political career.

The salvo from the pro-Gingrich “Winning Our Future” super PAC comes as Gingrich is vowing to step up his campaign-trail attacks on Romney, who bested former senator Rick Santorum (R-Pa.) by eight votes in Tuesday’s Iowa caucuses. Romney is hoping to ride that momentum into New Hampshire, which holds its first-in-the-nation primary next Tuesday, Jan. 10.

But first he’ll have to contend with an offensive by Gingrich, who trails in the New Hampshire polls but is returning fire after a pro-Romney super PAC ran an onslaught of negative ads against him in Iowa.

Gingrich’s campaign took out a full-page newspaper ad Wednesday in the New Hampshire Union Leader, an influential paper that endorsed Gingrich in November. The ad slams Romney as a “timid Massachusetts moderate.”

Polls show Romney leading his nearest opponent, Rep. Ron Paul (R-Tex.), by double digits in New Hampshire.

The move by Gingrich to use McCain’s words from four years ago against Romney may well be a preview of things to come in the next six days. While McCain’s endorsement of Romney is doubtless an influential one, there’s also the potential for the move to backfire, as other candidates on the trail remind primary voters of McCain’s previous attacks on Romney as well as of McCain’s own record as a Senate moderate.