Supporters of Sen. Claire McCaskill (D-Mo.) might be surprised to learn that among them is a billionaire behind a super PAC that has pledged to spend $10 million helping Mitt Romney get elected.

TD Ameritrade founder Joe Ricketts chairs Ending Spending Action Fund, a group that is backing GOP Senate candidates this cycle, with one notable exception: McCaskill, who Ricketts’s group endorsed shortly after the Missouri Senate primary in early August.

Sen. Claire McCaskill has an unexpected supporter. (Jeff Roberson/AP)

That’s the same Ricketts who helped propel Republican Deb Fischer to victory in the closing stage of the Nebraska GOP Senate primary. He also spent on behalf of Ted Cruz, the former solicitor general of Texas who ran to the right of Lt. Gov. David Dewhurst in the Lone Star State’s primary. (For more on Ricketts, a former Democrat with a daughter who is a bundler for President Obama, check out The Fix’s Aaron Blake’s May story on the billionaire.)

Why McCaskill? Ricketts’s super PAC, which was launched in 2010, is dedicated to reining in government spending. So it was attracted to the senator’s proposed earmark ban and her 2011 decision to sign on to a proposed measure to cap government spending at 20 percent of GDP in 10 years.

While Ending Spending has endorsed McCaskill, it hasn’t made any independent expenditures on her behalf so far. And Rep. Todd Akin’s (R) recent conversion from  advocating against curbing earmarks to now backing a ban may have complicated Ending Spending’s calculus in the race.

"We are proud to support Sen. McCaskill because of her strong record in favor of an earmark ban,” Ending Spending Action Fund President Brian Baker told The Fix. “However, since Akin has now apparently backed [an earmark ban], we are evaluating our options and we don’t discuss strategy with the news media.” Baker added that the group is not considering backing off its endorsement of McCaskill, but is evaluating its options with regard to spending money for her. 

Akin, the controversial Republican nominee who stoked controversy last month when he said in an interview that “legitimate rape” rarely causes pregnancy, received the endorsement of Sen. Jim DeMint (R-S.C.) and his Senate Conservatives Fund PAC this week, following his abrupt shift away from being an unapologetic earmarker.

So far, Ending Spending has only made independent expenditures on behalf of three Senate candidates: Fischer, Cruz, and Ohio Treasurer Josh Mandel (R). In addition to pledging to spend $10 million on behalf of Romney, Ricketts has also plans to spend $2 million electing Republicans to Congress, the Wall Street Journal recently reported.

McCaskill isn’t the first Democrat Ending Spending has gotten behind. It endorsed and spent money for then-Rep. Walt Minnick (D-Idaho) in 2010. Minnick was defeated.

Even if he is sold on McCaskill’s position on earmarks, there are secondary factors Ricketts might want to take under consideration as he weighs whether to use his super PAC to back McCaskill financially -- factors that could make him more unlikely to spend big for the first-term senator. 

As a Senate Democrat, McCaskill caucuses with Majority Leader Harry Reid (Nev.), a defender of earmarks who Ending Spending (which has a nonprofit sister organization) tried to defeat in 2010. What’s more, the Missouri race could well decide which party controls the Senate, and, as a result, whether Reid keeps his title as majority leader.

It remains to be seen whether Ending Spending will make expenditures on behalf of McCaskill or stay on the sidelines in a race that has been wildly unpredictable. But the group’s support for the Democrat is a reminder that politics can make for some unpredictable relationships. 

Updated at 5:08 p.m.