Outside groups backing Rep. Todd Akin’s (R-Mo.) bid for the U.S. Senate have spent more than $650,000 on the race since he made his "legitimate rape" comments, but the outside money is unlikely to make up for the immense spending gap he faces with Sen. Claire McCaskillClaire Conner McCaskillCalif. gov candidates battle for second place Senate panel advances Trump's CIA nominee Five votes to watch in fight over Trump's CIA nominee MORE (D).

Since Akin’s controversial comments broke in mid-August, GOP establishment figures have called for the candidate to drop out and have withdrawn funds from the race. But Akin has persisted, continuing his candidacy without the support of the national party and the big-dollar campaign funds that brings.

Akin is clearly feeling the pinch of his lone-wolf campaign: He raised about $1.6 million in the third fundraising quarter, less than a third of McCaskill’s $5.3 million haul. Akin posted only $553,000 cash on hand on September 30.

He’s had moderate support, however, from outside groups, which have invested more than $321,000 in independent expenditures opposing McCaskill and over $331,000 supporting Akin since his remarks went viral on Aug. 19. The bulk of the money has poured into the race this month, and all came after the deadline for Akin to drop out passed.

A number of the groups spending on the race haven’t been shy about their support for Akin.

Shortly after Sen. Jim DeMint (R-S.C.) endorsed the candidate, the PAC he founded, the Senate Conservatives Fund, pledged to support Akin with $290,000. The PAC has spent more than $54,000 in independent expenditures for Akin's e-mail list and online fundraising efforts, which have netted the candidate over $300,000 in small donations.

The National Right to Life PAC, which has endorsed Akin, spent nearly $28,000 on a newspaper advertisement and mailings, and after endorsing Akin, the Missouri Farm Bureau’s PAC spent nearly $20,000 on radio ads backing both Akin and Republican gubernatorial candidate Dave Spence.

Akin has also been endorsed by the National Rifle Association and the National Federation of Independent Businesses, both of which invested in the race. The NRA’s PAC poured in more than $210,000 and the NFIB spent $10,000 on mailings opposing McCaskill.

But still other groups have stayed largely silent on Akin’s continued candidacy, despite funneling money into the race.

Focus on the Family's PAC, CitizenLink, spent over $69,000 on mailings and social media advertising in support of Akin. But the group hasn’t endorsed him and declined to comment on their involvement in the race.

Freedom's Defense Fund PAC has spent the most supporting Akin -- $120,000 since late September on ad buys -- but the PAC doesn’t list Akin as an endorsed candidate on its website. It could not be reached for comment.

And Sen. Rand PaulRandal (Rand) Howard PaulOvernight Defense: Senate confirms Haspel as CIA chief | Trump offers Kim 'protections' if he gives up nukes | Dem amendments target Trump military parade Hillicon Valley: Lawmakers target Chinese tech giants | Dems move to save top cyber post | Trump gets a new CIA chief | Ryan delays election security briefing | Twitter CEO meets lawmakers Overnight Finance: Watchdog weighs probe into handling of Cohen bank records | Immigration fight threatens farm bill | House panel rebukes Trump on ZTE | Trump raises doubts about trade deal with China MORE’s (R-Ky.) PAC just released an ad attacking McCaskill for opposing legislation that would cut foreign aid to countries critical of the U.S., a $100,000 expenditure on the race. Paul’s spokesman said that the senator believes Akin is a better choice for Missouri voters than McCaskill, but the ad was released with little fanfare and did not come with an official endorsement from the senator.

The silence and small size of the expenditures from some of these groups reflects in part just how damaged Akin’s name remains within the Republican Party. After the National Republican Senatorial Committee (NRSC) disavowed Akin’s comments and the candidate himself, he became a political pariah, and few GOP figures have braved the backlash that would come with supporting his candidacy.

Those who have, however, may have motives beyond getting Akin elected. One GOP strategist following the race said that much of the support Akin has seen from outside groups might simply be politically motivated.

“Jim DeMint wants to be conservative kingmaker in the Senate. Rand Paul would consider a 2016 bid, or 2020 at that -- does this help him in a primary where he would probably struggle with social conservatives because he has more libertarian leanings, does this help him with the social conservatives?” the strategist said.

A week-long ad campaign in the state costs around $1 million, so small-dollar buys are unlikely to establish much of a message for the candidate.

Akin has also received the support of former GOP presidential candidates Newt Gingrich and Rick Santorum, both of whom ran as anti-establishment candidates at points during their campaign. Backing Akin could burnish those credentials if they eventually need to trot them out again.

And the more than $600,000 spent over the past three weeks of the campaign has done little to shift the race in his favor. Akin has lagged McCaskill in nearly every poll of the race since his comments broke, most recently by eight points.

McCaskill spent about $7 million in the third quarter, while Akin was only able to spend about $1.6 million during the same period. And the NRSC, though now outwardly supportive of the candidate, has consistently denied it plans to re-engage in the race. And as long as it remains out of the fray, few other big-spending GOP groups are likely to invest there.

--This post was updated at 4:05 p.m. on Oct. 22 to reflect the fact that Sen. Jim DeMint founded the Senate Conservatives Fund.