Comments about "legitimate rape" and human reproduction from U.S. Rep. Todd Akin (R-Mo.), GOP senate candidate for Missouri, has set off a firestorm of controversy and a growing chorus of Republicans calling on him to withdraw from the race. But the decision by Karl Rove's Crossroads GPS [3] to stop running ads in Missouri may be the most decisive call for withdrawal, and an indication of how well-funded outside groups are calling the shots in modern elections, to an even greater degree than political parties.

The Missouri senate race has been a top target for Crossroads GPS, one of the biggest 501(c)(4) nonprofit groups spending to influence elections without disclosing its donors. The $5.4 million already spent [4] on the race greatly outpaces the $2.2 million [5] raised by Akin thus far. Although Crossroads GPS is supposed to operate independently of the campaign, Akin's electoral success likely depends on Rove's group funding a barrage of ads that attack his Democratic opponent -- so the group's decision to drop its ads in the state is the strongest message yet that Akin should get out of the race.

Calls for Akin to Drop Out, and Paul Ryan Changes Tune

The controversy arose after remarks Akin made on Sunday justifying his blanket opposition to all abortion (even in cases of rape). He told a Kansas City television station on August 19 that "legitimate rape" usually does not lead to pregnancy, because "the female body has ways to try to shut that whole thing down." (A 1996 study [6] by the American Journal of Obstetricians and Gynecologists found that rape actually causes pregnancy with "significant frequency" and is the cause of over 32,000 unwanted pregnancies each year.)

Akin recently won a three-way Republican primary to challenge incumbent U.S. Sen. Claire McCaskill (D-Mo.), who is widely considered one of the most vulnerable Democratic senators and whose defeat is key to Republicans taking the Senate in 2012. McCaskill had been trailing Akin in the polls but his shocking comments may be a game-changer.

Almost immediately, conservative writers and politicians began calling [7] for Akin to drop out of the race before Tuesday's withdrawal deadline. The primary concern appeared to be that Akin's comments could hand Missouri's Senate seat to Democrats.

Reps. Todd Akin and Paul Ryan, Their Abortion Bill, and "Forcible Rape"

Even the GOP's presidential ticket, Mitt Romney and Paul Ryan, issued a joint statement saying they "disagree" with Akin's remarks and "would not oppose abortion in instances of rape." But as many pointed out [8], this represented a shift in policy for Ryan, who in the past had opposed abortion access for rape victims and worked closely with Akin in the House, even praising him [9] during the Missouri Senate primary.

In 2010 Akins and Ryan co-sponsored [10] a bill that would narrow access to abortion for rape victims receiving Medicaid (excluding pregnancies not arising from "forcible rape," such as cases of statutory rape or those caused when a woman is drugged or has limited mental capacity), and in 2009 the pair co-sponsored a bill [11] declaring a fertilized egg has the same legal rights as a human being, which not only would ban abortion but also turn many forms of birth control into a murder weapon [12].

Democrats have pounced [13] on the opportunity to tie the Romney-Ryan ticket to the controversy.

He who Controls the Pursestrings ...

But perhaps the most important voice signaling that Akin should resign is Karl Rove, whose dark money group Crossroads GPS [3] is pulling its ads in the Missouri race.

Crossroads recently booked an additional $874,000 in new ads [14] to start running Wednesday, on top of the $5.4 million already spent [4], putting the group on track to spend almost three times as much as Akins has raised so far. Crossroads GPS cancelled its recent ad buy on Monday. "The act speaks for itself," Crossroads spokesman Nate Hodson told Politico [15].

In a radio interview with Mike Huckabee on Monday, Akin said he regretted his comments but planned to stay in the race [16]. However, Akin's interview was recorded around the same time that Politico first reported [15] on the Crossroads GPS decision, so it is not clear whether Akin made his statement with the knowledge that Rove's operation would be withdrawing its support.

... Calls the Shots.

Along with Crossroads GPS, at least $15 million has been spent on the Missouri race so far by the Kochs' Americans for Prosperity [17], the U.S. Chamber of Commerce [18], and the 60 Plus Association [19]. The spending by these four groups, none of which disclose their donors, is greater than the $12 million [20] McCaskill has raised.

If any of these dark money groups join Crossroads GPS and stop running Missouri ads, Akin will almost certainly have a difficult time justifying his continued candidacy, given the lack of outside backing.

The role of independent outside groups is not unique to Missouri. Around the country, groups not working directly with the campaigns have been playing an increasingly powerful role in elections since the U.S. Supreme Court opened the floodgates for unlimited money in the decision Citizens United v. FEC [21]. Around half [22] of the half-billion-dollars spent so far in the 2012 campaign has come from outside groups; around half of that total [23] has been spent by groups that do not disclose their donors.

If Akin does drop out of the race on Tuesday, it will be another sign of the extraordinary power of these shadowy outside groups, particularly secretly-funded operations like Crossroads GPS, in the post-Citizens United world. As the saying goes, he who controls the purse strings can call the shots.