McCaskill is facing serious opposition in her reelection fight, with conservative groups making her a top target for the fall elections. GOP super-PAC Crossroads GPS is already on the air with ads against her and spending $2 million in the state this month alone.

McCaskill has strongly condemned the interference of outside groups in elections, in part because they do not have to disclose their donors. The senator will have no say over the super-PAC, because such groups cannot be affiliated with a candidate, but it might be awkward for her to get help from the same type of group she wants to eliminate.

The Missouri Republican party was quick to point out that potential for awkwardness.

“Chameleon Claire has repeated feigned outrage over what she has labeled ‘spooky PACs.’ But now that another of these organizations is spending money on her behalf, McCaskill’s silence is deafening," state GOP Executive Director Lloyd Smith said in a statement. "If McCaskill is serious about campaign finance transparency, then she will condemn her allies as vigorously as she condemns her opponents. We’re not holding our breath.”

The group's creator, Richard Martin, who is now a lobbyist but ran McCaskill's 2006 campaign, told the Springfield News-Leader that the super-PAC was created to help "moderate Democrats ... not necessarily" just McCaskill.

He estimated that the group, which does not have to file a disclosure report until next quarter, has raised about $100,000 so far.

McCaskill's campaign called on the super-PAC to reveal its donors.

"Claire is adamant — outside groups need to disclose their donors and the same goes for groups looking to support her, if not more so,” said McCaskill for Missouri spokesman Erik Dorey. “Whether they're secret money groups that are never required to disclose, or a group like this one that has quarterly reporting requirements, Claire is making herself crystal clear: disclose your donors."

McCaskill also recently bought up $3 million worth of TV advertising time for October. She will face one of three Republicans — Todd Akin, John Brunner and Sarah Steelman — vying for the nomination in the Aug. 7 Missouri primary.

— Last updated at 5:49 p.m.