(CNN) -- Republican Rep. Todd Akin allowed a state deadline to pass Tuesday, defiantly staying in the race to unseat Democratic Sen. Claire McCaskill despite mounting calls for him to withdraw over incendiary remarks on rape and pregnancy.
Under Missouri law, Akin would now need to get a court order to pull out of the race as he waited beyond the 5 p.m. deadline. He would also be required to pay for any necessary reprinting of ballots.
But Akin said he has no plans to drop out. He 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 his own Republican Party, congressional colleagues and others to step aside.
"I'm in this race for the long haul and we're going to win it," Akin told conservative radio host Dana Loesch.
He spoke minutes after five past and present Republican senators from Missouri, including highly regarded figures John Danforth and Christopher "Kit" Bond, added their voices to widespread calls for Akin to end his campaign.
"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."
Republican presidential candidate Mitt Romney also advised Akin to end his campaign, saying: "His fellow Missourians urged him to step aside, and I think he should accept their counsel and exit the Senate race."
Romney's running mate, Rep. Paul Ryan of Wisconsin, echoed the call, according to a statement from his spokesman.
Other prominent Republicans to join the chorus urging Akin's withdrawal included Wisconsin Gov. Scott Walker and veteran Sen. Kay Bailey Hutchison of Texas.
Akin complained his detractors overreacted to a liberal media campaign to take him down. He said fellow Republicans "ran for cover at the first sound of gunfire."
His decision means he faces the first statewide race of his career with no mainstream GOP backing. After he announced his intention to stay in the race, the National Republican Senatorial Committee made clear it would not provide any help.
"We continue to hope that Congressman Akin will do the right thing for the values he holds dear, but there should be no mistake -- if he continues with this misguided campaign, it will be without the support and resources of the NRSC," said a statement by communications director Brian Walsh.
The political drama exposes the tension in the Republican Party created by the growth of the conservative movement, particularly the birth of the tea party movement before the 2010 mid-term elections.
A six-term congressman who won more than 60% support in his five re-election efforts, Akin is a staunch conservative Christian who opposes abortion. By staying in the race, Akin's candidacy ensures the abortion issue will be a focal point of next week's Republican National Convention.
Romney is basing his presidential election campaign on economic issues, and the attention to social issues such as abortion distracts from his message that he is better qualified than President Barack Obama to restore strong growth and create jobs.
The controversy erupted after Akin told a television interview Sunday that a woman's body is capable of preventing pregnancy in cases of "legitimate rape." He later apologized, explaining he meant to say "forcible rape" and acknowledged that women "do become pregnant" during such instances.
Romney's 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 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.
The issue is particularly sensitive for Romney's running mate, Ryan, who is a devout Catholic opposed to abortion. The Romney campaign acknowledged that Ryan personally opposes abortion in cases of rape, but said Romney's view is the policy of the ticket.
In a new ad and radio interviews Tuesday, Akin called his mistake a lone misstep with unintended consequences.
"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."
He told the radio show of former Arkansas Gov. Mike Huckabee that "the defense of the unborn and a deep respect for life which underlie all of America -- those are important parts of who we are and they are not things to run away from."
Akin also characterized his mistake as isolated and relatively minor.
"Well it just seems that I just misspoke one word in one sentence on one day," he told Huckabee. "I hadn't done anything that was morally or ethically wrong, as sometimes people in politics do. ... It does seem like a little bit of an overreaction."
Later, he said on the radio show of Sean Hannity that he sees no harm in continuing to remain in the race.
"My interest in this race has nothing to do with me. It has to do with who we are as a nation," he said. "I think it will help Romney and I think it's going to help the Republican party."
His decision won support from the Missouri Republican Assembly, which issued a statement urging the party to back Akin in the battle against McCaskill.
"The Republican leadership needs to grow a spine and disallow the Democrats, who always support their candidates even when they are wrong, to dictate our stance," the group's statement said. "... While Todd may have been indiscreet in his word choice, he was not wrong in his facts. Todd can win despite this misstep. All Republicans will lose if they continue throwing their candidates under the bus because of a poor word choice."
However, top Republicans strengthened their calls for Akin to depart the race.
"In his heart of hearts, I'm certain that he is sincerely sorry for what he said, but in this instance, when the future of our country is at stake, sorry is not sufficient," Senate Republican leader Mitch McConnell said in a statement. "To continue serving his country in the honorable way he has served throughout his career, it is time for Congressman Akin to step aside."
American Crossroads, an influential conservative super-PAC, also cited the importance of the election, in which McCaskill's seat is considered vital to GOP hopes of gaining a Senate majority.
"Rep. Akin faces a simple choice: Will he help Democrats hold the McCaskill seat and potentially the Senate majority by staying in the race, or will he help Republicans defeat Barack Obama's most reliable ally in the Senate by getting out?" said Steven Law, the group's president and CEO.
Nate Hodson, a spokesman for the Crossroads GPS non-profit that backs Republican candidates around the country, said the group won't spend any more money in the Missouri Senate race for now.
Akin told Huckabee that he still was ahead of McCaskill in the polls, 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."
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."
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.
If Akin were to drop out of the race, the state Republican Party would choose another candidate to run against McCaskill, considered one of the most politically vulnerable senators in the country.
Democrats have sought to portray Akin's stance on rape and abortion as indicative of the broader Republican approach to women's issues.
Obama said Akin's remarks were "offensive" and didn't make sense, while House Democratic leader Nancy Pelosi called them "another manifestation of the total disregard and disrespect of women by Republican leaders."
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.
"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."
Akin was one of the first members of Congress to join the Tea Party Caucus in 2010 and has easily won re-election 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."
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CNN's Peter Hamby, Paul Steinhauser, Kevin Liptak, Dana Bash, Chris Welch, Shawna Shepherd and Ashley Killough contributed to this report.