The Obama campaign launched its latest attack ad Tuesday, linking Mitt Romney to Donald Trump's "birther" comments and suggesting the presumptive Republican nominee is allowing "voices of extremism" to flourish in the party.
The video was released on the same day that Romney plans to hold a fundraiser in Las Vegas with Trump, who toyed with a run for president before backing off and eventually endorsing Romney.
The real estate mogul and reality TV star continues to raise questions about Obama's birthplace, most recently bringing up spurious claims that he was born in Kenya.
Romney, asked about Trump's remarks on Monday, said "I don't agree with all the people who support me and my guess is they don't all agree with everything I believe in."
He added, "I'm appreciative to have the help of a lot of good people."
The Obama campaign video tried to contrast Romney with 2008 GOP presidential nominee Sen. John McCain. The ad opened by saying McCain "stood up" to the "voices of extremism" in the party -- and played clips of McCain scolding a supporter who said he was scared of Obama, and another who voiced concern that he was an "Arab."
"Why won't Mitt Romney do the same?" the video asked, before playing a string of clips of Trump interviews in which he discussed Obama's birth certificate. "McCain and Romney. Two Republican nominees. Only one willing to lead."
The campaign supplemented the video with a statement Tuesday afternoon saying Romney "lacks the backbone to stand up to a charlatan" like Trump.
The video follows a pattern of negative ads from the Obama campaign. Campaign aides, and Obama himself, have defended spots highlighting companies that went under following the involvement of Romney's former private equity firm Bain Capital.
One of the early hard-hitting videos from the Obama campaign also questioned whether Romney, if he were president, would have green-lighted the raid on Usama bin Laden's compound.
Romney is expected to clinch the GOP nomination Tuesday once Texas holds its primary.