A conservative advocacy group is spending $50,000 on an ad buy targeting Sen. Claire McCaskill (Mo.), considered one of the most vulnerable Democrats in the 2012 cycle.

American Crossroads, the 527 group advised by GOP strategist Karl Rove that played a major role in the 2010 elections, hits McCaskill as a “big spending liberal” and lashes out at her vote in favor of the 2009 stimulus bill.

The ad accuses McCaskill of “using a body-double to cast her votes in Washington, D.C.”

American Crossroads valued the ad buy at $50,000. The group also launched the website TruthAboutClaire.com, which also focuses on McCaskill’s fiscal policy record.

A Public Policy Poll conducted at the beginning of May showed McCaskill in a tight race against potential Republican challengers.

According to the Missouri Democratic Party, McCaskill is comfortable being "the underdog" in this race.

"All of the attacks against Claire are going to be about distorting her record. As an independent who has cut spending by going after the big contractors, she's stepped on some pretty big toes, and there are quiet a few giants who want to take her out as a result," said Caitlin Legacki, senior spokeswoman for the Missouri Democrats.

Only two candidates — former state Treasurer Sarah Steelman and Rep. Todd Akin (R-Mo.) — have so far declared that they will run for the Republican nomination in the Missouri Senate race.

--Updated at 3:49 p.m.