ST. LOUIS -- Rep. Todd Akin vowed to fight on in his embattled Senate campaign, but a significant deadline loomed Tuesday that was bound to intensify pressure on the Missouri congressman to abandon the race over his comments that women's bodies can prevent pregnancies in cases of "legitimate rape."

Akin has been frantically trying to salvage his once-promising bid against incumbent Democrat Claire McCaskill in a race long targeted by the GOP as crucial to regaining control of the Senate. But ominous signs were mounting against the six-term legislator from suburban St. Louis, most notably the apparent loss of millions of dollars in campaign advertising money.

Early Tuesday Akin posted an apology video online, but made no mention of the race. He went on two conservative radio shows Monday, pledging to keep the campaign alive, even as some people in his own party urged him to step aside.

The decision has some urgency. Missouri election law allows candidates to withdraw 11 weeks before Election Day. That means the deadline to exit the Nov. 6 election is 5 p.m. Tuesday. Otherwise, a court order would be needed to remove a name from the ballot.

"I was told the decision has to be made by 5 tomorrow, but I was calling you and letting you know that I'm announcing today that we're in," Akin told radio host Sean Hannity.

In a radio interview with former Republican presidential candidate Mike Huckabee, Akin repeatedly apologized for the remarks but also vowed to stay in the race.

"The good people of Missouri nominated me, and I'm not a quitter," Akin said.

The uproar began Sunday, when St. Louis television station KTVI aired an interview in which Akin was asked if he would support abortions for women who have been raped.

"It seems to me, first of all, from what I understand from doctors, that's really rare. If it's a legitimate rape, the female body has ways to try to shut that whole thing down," Akin said.

Later Sunday, Akin released a statement saying that he "misspoke" during the interview.

In the interviews with Huckabee and Hannity, he apologized repeatedly, acknowledging that rape can lead to conception.

"Rape is never legitimate. It's an evil act. It's committed by violent predators," Akin said. "I used the wrong words the wrong way."

But the damage had already been done. The comments drew a sharp rebuke from fellow Republicans, including presumptive presidential nominee Mitt Romney and his vice presidential choice, Rep. Paul Ryan of Wisconsin.

The Senate's top Republican said Akin's comments about rape might "prevent him from effectively representing" the Republican Party. Mitch McConnell called on Akin to "take time with his family" to consider whether he should continue in the Missouri Senate race.

Two other Republican senators, Scott Brown of Massachusetts and Ron Johnson of Wisconsin, urged Akin to resign.

Akin also apparently lost a key source of funding. Sen. John Cornyn, head of the National Republican Senatorial Campaign Committee, told Akin that $5 million in advertising set aside for Missouri will be spent elsewhere and that Akin will get no other help from the committee, according to a committee official who spoke on condition of anonymity because the conversation was private.

Cornyn told Akin that he was endangering the GOP's hopes of getting a Senate majority by staying in the race, the official said.

At least one outside group that has pounded McCaskill with ads, the Karl Rove-backed Crossroads organization, also pulled its ads from Missouri.

In an apparent effort to claw back some of that funding, Akin posted a video to YouTube early Tuesday in which he described himself as a compassionate father of two daughters, apologized for his poor choice of words and clarified that he understands the possible outcome of rape.

"Fact is, rape can lead to pregnancy. The truth is rape has many victims. The mistake I made was in the words I said, not in the heart I hold. I ask for your forgiveness," he said.

President Barack Obama said Monday that Akin's comments underscore why politicians - most of whom are men - should not make health decisions on behalf of women.

"Rape is rape," Obama said. And the idea of distinguishing among types of rape "doesn't make sense to the American people and certainly doesn't make sense to me."

It was just two weeks ago that Akin was at the top of the political world in Missouri after winning a hotly contested three-way battle with millionaire businessman John Brunner and former state Treasurer Sarah Steelman for the right to challenge McCaskill in the November election. Missouri has grown increasingly conservative in recent years, and McCaskill is seen as vulnerable.

She was not among those calling for her opponent to get out of the race.

"What's startling to me is that (Republican) Party bigwigs are coming down on him and saying that he needs to kick sand in the face of all the primary voters," McCaskill said at a campaign event Monday in suburban St. Louis. "I want Missourians to make a choice in this election based on policy, not backroom politics."

Names are being floated about a possible replacement for Akin. A favorite is Tom Schweich, the state auditor who was courted to run for Senate earlier this year but declined.

Other names mentioned include former Sen. Jim Talent, who lost to McCaskill in 2008; former Gov. Matt Blunt, the son of Missouri's other senator, Roy Blunt; two members of Missouri's House delegation, Blaine Luetkemeyer and Jo Ann Emerson; and Akin's two unsuccessful primary opponents, Brunner and Steelman.

Talent, who lost his seat to McCaskill in 2006, said Monday he had been asked to run but replied: "I'm not running for the Senate."

"I'm totally ruling it out," Talent said in Tampa, Fla.

University of Missouri-St. Louis political scientist Dave Robertson said any candidate who might replace Akin would face significant challenges so close to the election.

"You're going to be on the defensive and starting from behind with a very short time to go," Robertson said.

Missouri has faced awkward situations in Senate elections before. In 2000, Democratic candidate Mel Carnahan died in a plane crash three weeks before the November election. His name remained on the ballot, and he defeated Republican incumbent John Ashcroft.

Carnahan's widow, Jean, served for two years before losing in a special election to Talent.

If Akin were to leave, state law gives the Republican state committee two weeks to name a replacement. The new candidate must file within 28 days of Akin's exit.

The American Congress of Obstetricians and Gynecologists said a woman who is raped "has no control over ovulation, fertilization or implantation of a fertilized egg (i.e., pregnancy). To suggest otherwise contradicts basic biological truths."

Between 10,000 and 15,000 abortions nationwide occur each year among women whose pregnancies resulted from rape or incest. An unknown number of babies are born to rape victims, the group said.

Research on the prevalence of rape and rape-related pregnancies is spotty. One estimate published in 1996 said about 5 percent of rapes result in pregnancy, or about 32,000 pregnancies among adult women each year.

Still, the idea about rape and pregnancy has been raised in anti-abortion circles for at least three decades.

Leon Holmes, onetime head of Arkansas Right to Life, wrote in a 1980 letter to a newspaper that concern for rape victims "is a red herring because conceptions from rape occur with the same frequency as snow in Miami." Holmes went on to become a federal judge.

Abortion foes in the Pennsylvania and North Carolina legislatures have made similar statements. And in Arkansas in 1998, Republican Senate candidate Fay Boozman came under fire for saying pregnancies from rape were uncommon. He apologized and later acknowledged that his unsuccessful campaign never recovered from the criticism. He died in 2005.