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."