(CNN) -- A defiant Rep. Todd Akin of Missouri said Tuesday he will stay in the Senate race to unseat Democrat Claire McCaskill despite mounting calls from across the political spectrum for him to withdraw over incendiary comments on rape and pregnancy.
Akin cited what he called a grassroots conservative movement in the country that needs a voice in government for his decision to reject increasing pressure from the Republican Party, congressional colleagues and others to step aside.
"There is a cause here," Akin said on former Arkansas Gov. Mike Huckabee's radio program. "There is a part of the message that's missing and a lot of people feel left out of the parties."
He spoke minutes after five past and present Republican senators from Missouri, including highly regarded names such as John Danforth and Christopher "Kit" Bond, added their voices to calls for Akin to get out of the race.
"We do not believe it serves the national interest for Congressman Todd Akin to stay in this race," said the statement by Sen. Roy Blunt and former senators Danforth, Bond, John Ashcroft and Jim Talent. "The issues at stake are too big, and this election is simply too important. The right decision is to step aside."
The latest call signaled that Akin stood alone in wanting to remain in the race.
He told Huckabee in Tuesday's interview that a new poll showed him still ahead of McCaskill and that pundits had wrongly predicted he would lose in the GOP primary campaign that chose a Senate nominee for the November election.
"We have a message that people understand," Akin said. "It isn't something that's in the brain. It's in the heart."
He also called the response to his "one mistake" an overreaction.
Top Republican officials -- including Senate GOP leader Mitch McConnell and Republican National Committee Chairman Reince Priebus -- also have signaled that the six-term Missouri congressman should withdraw.
Under Missouri law, Akin would require a court order to do so if he waits beyond 6 p.m. ET on Tuesday.
Akin released a new television ad on Tuesday that seeks forgiveness for what he called "serious" mistakes in his Sunday remarks about "legitimate" rape.
However, Akin faces calls from all sides, including the tea party movement that he championed, to step aside.
Responding to Akin's request for forgiveness, Tea Party Nation website blogger Judson Phillips wrote Tuesday that while everyone makes mistakes, this particular error was politically fatal.
"Just because we forgive him for doing something stupid does not mean we should now support his idiotic decision to try and continue his campaign," Phillips wrote. "There are two kinds of mistakes politicians can make. There are those that are recoverable and those that are not. At least for this election, Akin's mistake is not a recoverable one."
Nate Hodson, a spokesman for the Crossroads GPS non-profit that backs Republican candidates around the country, said Tuesday the group won't spend any more money in the Missouri Senate race for now.
Voters sounded split over the controversy in Akin's U.S. House district, which comprises a broad swath of suburban St. Louis including formerly rural areas that have seen a large population increase in recent decades.
Gene Wood of St. Charles, about 20 miles west of St. Louis, said he voted for Akin in past elections and the recent Senate primary and still plans to support him.
"It strikes me that this is a tempest in a teapot," Wood said, calling the issue "a matter of semantics."
To Wood, "people are looking for something to accuse Todd or drag him down. ... I don't think he said anything that's really worth all the coverage that it's getting."
Another St. Charles resident, Judi Meredith, owns a counseling business that deals with rape victims. She has not supported Akin in the past and said she was horrified by his comments.
"It really showed a level of lack of knowledge and ignorance about society and rape as it's used against women as a tool of oppression," Meredith said, adding she believes Akin should withdraw.
Opinion: Wake up -- it's not just Akin
In an interview on Sunday, Akin said a woman's body is capable of preventing pregnancy in cases of "legitimate rape." He later apologized, saying he was referring to "forcible rape" and acknowledged that women "do become pregnant" during such instances.
"Rape is an evil act; I used the wrong words in the wrong way. And for that, I apologize," he said in the new ad that came out Tuesday. "As the father of two daughters, I want tough justice for predators, have a compassionate heart for the victims of sexual assault, and I pray for them. Fact is, rape could 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."
If Akin stays in the race after the Tuesday night deadline, he must get a court order and pay for any necessary reprinting of ballots. The state Republican Party would choose another candidate to run against McCaskill, considered one of the most vulnerable senators in the country.
Empty chair on Piers Morgan after Akin cancels appearance
After previously criticizing Akin's remarks, McCaskill issued more supportive comments later Monday.
"While I disagree with what he said, he has now in the last few hours really apologized for what he said," McCaskill noted. "I think what is startling to me is that these party bigwigs are coming down on him and saying that he needs to kick sand in the face of the Republican primary voters."
Amy Kremer, chairwoman of the Tea Party Express, said Tuesday that "the fact that Claire McCaskill wants him to stay in the race speaks volumes."
She said the longer Akin remains in the race, the more his comments will overshadow crucial economic issues.
Akin's remarks deemed 'harmful' to rape survivors
"What he said is completely indefensible and changes the narrative of what's going on here," Kremer said Tuesday on CNN's "Starting Point with Soledad O'Brien." "If he looks at the bigger picture, he will do what's best and step aside."
Priebus called on Akin to exit the race, even urging the congressman to stay away from the GOP convention.
"I would prefer that Todd Akin do the right thing for our party and our candidates, and I would prefer him not come," Priebus said Monday on CNN's "Erin Burnett OutFront."
Akin asks for 'forgiveness' in new ad
The National Republican Senatorial Committee has advised Akin that it will not support his campaign if he stays in the race, a source from the group told CNN.
Akin's comments shifted the political debate to the abortion issue, much to the disappointment of certain GOP presidential nominee Mitt Romney. His campaign quickly distanced the candidate from Akin and declared that Romney didn't oppose abortion in cases of rape.
However, the party platform being drafted ahead of next week's Republican National Convention includes an endorsement of a "human life amendment" to the Constitution that would outlaw abortion with no explicit exemption for rape or incest.
The language, approved by the platform committee on Tuesday, is similar to the platforms that were adopted by the party at its conventions in 2000, 2004 and 2008. Convention delegates are expected to approve the platform next week.
Romney told National Review Online on Monday that the comments by Akin were "insulting, inexcusable and, frankly, wrong."
President Barack Obama said Akin's remarks were "offensive" and didn't make sense.
Opinion: Pregnant by rape? Of course
The issue is particularly sensitive for Romney's running mate, Rep. Paul Ryan of Wisconsin, who is a devout Catholic opposed to abortion. The Romney campaign acknowledged that Ryan personally opposed abortion in cases of rape, but said Romney's view was the policy of the ticket.
Meanwhile, a last-minute decision by Akin to cancel a prime-time TV appearance Monday on CNN's "Piers Morgan Tonight" created a flurry of comments on Twitter. Instead of Akin, viewers saw an empty chair.
McCaskill also was booked to appear on the show, but canceled earlier in the day.
Republicans consider McCaskill, first elected in 2006, highly vulnerable in her bid for a second term. The Missouri race also is considered vital to GOP hopes of regaining a majority in the Senate.
Akin was one of the first members of Congress to join the Tea Party Caucus in 2010 and has easily won reelection in recent years. The lawmaker raised $2.2 million this cycle, as of July 18.
Before the new controversy, the top nonpartisan political handicappers had rated the Missouri race a "toss-up."