Missouri Sen. Claire McCaskill is airing a new ad highlighting Rep. Todd Akin's much-criticized remark about rape, moving quickly to capitalize on her Republican challenger's comment as it became clear Tuesday that he wasn't dropping out of the Senate race.

Akin also sought to capitalize on Tuesday's deadline for candidates to withdraw by court order from the Missouri ballot. Instead of quitting, the congressman rallied with about 200 supporters in St. Louis before launching a statewide bus tour and asked for donations to replenish his financially strapped campaign.

Akin has repeatedly apologized and rejected calls from top Republicans to quit the Senate race after a television interview aired Aug. 19 in which he said that women's bodies have ways of averting pregnancy in cases of what he called "legitimate rape."

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Some doubted his resolve to remain in the race, making Tuesday's drop-out deadline an important mark six weeks before the Nov. 6 election. But the 5 p.m. deadline passed without Akin quitting.

At his St. Louis rally, Akin said he was "given a trust" by voters who nominated him in the Aug. 7 Republican primary. He also compared himself to former Missouri Sen. Harry Truman, who overcame opposition from fellow Democrats to win re-election in 1940, later becoming vice president and then president.

"I have one purpose going into November, and that's replacing Claire McCaskill," Akin said in a five-minute speech prefaced by endorsements from various religious leaders and conservative activist Phyllis Schlafly. He left without taking questions from reporters.

Earlier Tuesday, McCaskill's campaign website debuted an ad citing comments Akin made over the past year expressing dislike for Social Security, questioning the constitutionality of the Medicare program and voicing opposition to the federal government's role in setting a minimum wage and financing student loans.

The ad culminates by referencing Akin's remark about rape and asks: "What will he say next?"

McCaskill's campaign said the ad is running on TV stations statewide.

It marks the first time McCaskill has addressed Akin's rape remark in an ad, though she has cited his comments about other issues from the moment he won the Republican primary. As the drop-out deadline passed Tuesday, McCaskill also launched a new online fundraising drive with the declaration: "Todd Akin is Officially Claire's Opponent."

After his rape remark, Akin lost the financial support of the Republican National Committee, the GOP's Senate campaign arm and the Crossroads group affiliated with Republican strategist Karl Rove. That eliminated millions of dollars of planned advertising in Missouri.

Akin has countered that with a small-dollar, online fundraising drive highlighting how he's standing up to Republican Party bosses and President Barack Obama's administration. Former Republican presidential candidate Newt Gingrich headlined a more traditional fundraiser Monday for Akin. And late Tuesday,

And late Tuesday, Missouri U.S. Sen. Roy Blunt changed course by saying he was supporting Akin — whom he'd called on to quit the Senate race last month in a joint statement with four former Missouri senators.

"Congressman Akin and I don't agree on everything, but he and I agree the Senate majority must change," Blunt said in a statement. "From Governor Romney to the county courthouse, I'll be working for the Republican ticket in Missouri, and that includes Todd Akin."

Earlier Tuesday, the Freedom's Defense Fund said it was coming to Akin's aid by spending $250,000 for ads on cable TV, radio and the Internet that will begin running Thursday. The group's website says it endorses "conservative stalwarts" who support gun rights, oppose abortion, cut taxes and want to secure the nation's borders.

"We stand for conservative candidates when other people have turned their backs on them," said Kevin Sawyer, a spokesman for the group.

Separately, the Senate Conservatives Fund sent a survey to thousands of people on its email list Tuesday asking whether it should support Akin and, if so, how much money people would be willing to donate. Although it had previously remained neutral, the organization cited the fact that Akin is now fixed on the ballot and still has a shot of defeating McCaskill. Republicans need a net gain of four seats to win control of the Senate from Democrats.

The Senate Conservatives Fund opposes earmarks, which Akin has used and defended during the Senate primary. Akin obtained several for the 2010 fiscal year — the last year earmarks were funded in appropriations bills — including $800,000 for two highway local projects and $2.4 million for "air crew body armor and load" purchased from Eagle Industries, headquartered in Fenton.

The Fund's executive director and Akin said this week that they both support an earmark ban.

But Akin is continuing to get the cold shoulder from other groups that often aid Republican candidates. The Club for Growth and the U.S. Chamber of Commerce both reaffirmed Tuesday that they have no plans to get involved in Missouri's Senate race.

Republican National Committee Chairman Reince Priebus flatly reiterated Sunday he would be sending no resources to aid Akin's campaign. And on Tuesday, two knowledgeable Republican officials told The Associated Press there was no chance the national GOP would spend any money on Akin in the short term. The officials spoke on condition of anonymity because they weren't authorized to discuss sensitive matters.

But the Missouri Republican Party pledged to stand behind Akin's campaign.

"We are confident that Todd will defeat McCaskill in November, and the Missouri Republican Party will do everything we can to assist in his efforts," said state GOP Chairman David Cole.

And there are other ways to pump Republican money into the Missouri Senate race. Come late October, if Senate control isn't within reach and Akin still has a fighting chance, individual senators, for example, could use their own political action committees to try to nudge Akin over the line.

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Associated Press writers Jim Salter in St. Louis, and Andrew Taylor and Laurie Kellman in Washington, D.C., contributed to this report.

Copyright 2012 The Associated Press.