Mitt Romney added firepower Tuesday to Republican demands that embattled Senate candidate Rep. Todd Akin withdraw from the race, saying that Akin should accept the counsel of “his fellow Missourians.”

In his first explicit call for Akin’s exit after the Missouri congressman used the phrase “legitimate rape” when talking about abortion and pregnancy, Romney said: “As I said yesterday, Todd Akin’s comments were offensive and wrong and he should very seriously consider what course would be in the best interest of our country. Today, his fellow Missourians urged him to step aside, and I think he should accept their counsel and exit the Senate race.”

Fighting back against enormous pressure by other Republican leaders to withdraw, Akin earlier Tuesday accused his critics of “overreaction” and used a radio interview to turn his campaign into a cause for “the regular people” against “the big party people.”

Akin’s remarks on rape and abortion have incited fury among liberals and conservatives alike. But in the afternoon interview on Mike Huckabee’s radio show, Akin said he would stick to his decision to stay in the race against Sen. Claire McCaskill (D).

“I’ve had a chance now to have run through a primary, and the party people said when you win the primary then we’ll be with you. Well, they were with us. Then I said one word and one sentence on one day, and everything changed,” Akin told Huckabee, an early supporter. “I haven’t done anything morally or ethically wrong. It does seem like a little bit of an overreaction.”

He then went on to liken his decision to a type of crusade. “We believe taking this stand is going to strengthen our country — going to strengthen, ultimately, the Republican Party,” he said. “What we’re doing here is standing on a principle of what America is.”

Akin said that his supporters and “good friends, closer than brothers,” had asked him to stick it out. He added that he has received “continuing calls from other congressmen” expressing their support. (He did not name any of these congressmen.)

He compared his race to the GOP primary, when he was outraised by rivals and lacked institutional backing. And he referred to the potential to attract more independent voters. “I realize that there are now a lot of other bravehearts that don’t fit into the political parties exactly,” he said. “I believe there is a cause here, and there is a part of the message that’s missing, and a lot of the people feel left out of the parties.

“What we’re seeing right now is a tremendous outpouring of support from just regular small people,” he said. “They’re not the big party people.”

Republicans were hoping Akin would heed their calls to withdraw from the race and preserve the party’s chances to take back the upper house. Akin’s interview drew quick rebuke from those ranks, citing potential harm to the party’s election chances.

The conservative super PAC American Crossroads said in a statement that “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?”

Rep. Sean Duffy (R-Wis.) also referred to the potential damage Akin could do to the Senate race. “As a father and a former prosecutor who defended victims of rape, I strongly denounce Rep. Todd Akin’s callous and offensive remarks. A crime as violent and heinous as rape should never be minimized, especially by a member of Congress,” Duffy said in a statement. “I repudiate his comments and call for him to step aside so the people of Missouri can put forth a viable candidate who can defeat Claire McCaskill in November.”

Akin has said repeatedly that he has no intention of ending his campaign, even as his prospects of winning have probably been diminished with Republican leaders pulling financial support from the contest and denouncing his comments.

Tuesday morning, with a 5 p.m. deadline looming for stepping aside without having to seek a court order, Akin released a new campaign commercial called “Forgiveness.”

“Rape is an evil act,” he says in the 30-second ad. “I used the wrong words in the wrong way, and for that I apologize.”

According to reports, the spot is part of a $150,000 ad buy set to run through Aug. 27, suggesting that Akin has dug in his heels, at least for now.

If Akin decides to withdraw, Republicans can select a replacement. But if he remains a candidate, he would have until Sept. 25 to petition a court to be removed from the ballot if he changed his mind.

After Sept. 25, Missouri candidate names remain on the ballot, even in the event of death.

GOP leaders have been putting heavy pressure on Akin to leave the race, with Romney earlier calling Akin’s comments “insulting, inexcusable and, frankly, wrong.”

“His comments about rape were deeply offensive. I can’t defend what he said; I can’t defend him,” Romney said on a interview with WMUR on Monday. “The thing he should consider is what’s in the best interest of the things he believes most deeply. What will help the country at this critical time.”

Sen. Kelly Ayotte (R-N.H.) tweeted Tuesday: “I agree with @JohnCornyn for cutting off $. Akin should step aside now.”

Senate Minority Leader Mitch McConnell (Ky.) and Sens. John Cornyn (Tex.) and Ron Johnson (Wis.) have also asked Akin to step aside, as has Republican National Committee Chairman Reince Priebus.

The National Republican Senatorial Committee released a statement Tuesday saying that they continued to hope that Akin bows out, noting that McCaskill has not called for him to quit.

“This is undoubtedly a difficult time for Congressman Akin, but the stakes in this election are far bigger than any one individual,” said Brian Walsh, NRSC spokesman, in a statement. “By staying in this race, Congressman Akin is putting at great risk many of the issues that he and others in the Republican Party are fighting for, including the repeal of ObamaCare.”

The controversy has become a distraction to Republican leaders as they prepare for their Tampa convention next week and finalize the party’s platform, which will include language supporting a ban on abortion except when the mother’s life is in danger, as first reported by CNN.

Democrats have labeled the party’s platform, which is similar to the abortion language the GOP used in 2004 and 2008, the “Akin Plank.”

In the wake of Akin’s comments, the Romney campaign released a statement Sunday saying that Romney and vice presidential running mate Rep. Paul Ryan (Wis.) would not oppose abortion in instances of rape.

As Ryan prepared to address a crowd of more than 1,000 Tuesday morning at a steel supply factory near Pittsburgh, a sign greeting his motorcade underscored the messaging challenge that Akin brings to the national ticket:

“Ryan and Akin Agree: Only Some Rapes Count,” read a handmade poster, which was held by one of two dozen protesters down the street from Beaver Steel Services.

Democrats have seized on the comments, aiming to tie Akin to Ryan, and widen the gender gap — Obama has a 22-point favorability advantage over Romney when it comes to women.

In 2011, Ryan, Akin and other GOP candidates co-sponsored a bill that would have strengthened federal prohibitions on abortion funding, redefining rape so that only “forcible rape” would be exempt from the restriction.

Ryan also joined Akin in co-sponsoring a “personhood bill” in 2009 which would grant legal rights to embryos.

President Obama weighed in Monday with a surprise appearance in the press briefing room at the White House.

“Rape is rape, and the idea that we should be parsing and qualifying and slicing what types of rape we are talking about doesn’t make sense to the American people and certainly doesn’t make sense to me,” Obama said. “So what I think these comments do underscore is why we shouldn’t have a bunch of politicians, a majority of whom are men, making health-care decisions on behalf of women.”

The Missouri race has been a priority for Republicans who see the seat as a must-win for their chances of taking the Senate.

Republicans need four seats to claim the majority — or three if Romney defeats Obama, giving Ryan the tie-breaking vote — and have long viewed McCaskill as the most vulnerable Democrat running for reelection.

A Public Policy Polling survey released late Monday showed Akin with a slight lead over McCaskill, 44 percent to 43 percent.

Yet Democrats have long viewed Akin as their best chance to retain the seat because of his conservative views.

Missouri has increasingly tilted away from Democrats since McCaskill’s 2006 victory. In the days leading up to Akin’s Aug. 7 primary victory, GOP strategists issued memos claiming that any of the three contenders would defeat McCaskill, but senior advisers made it clear they preferred either of the two alternatives to Akin: John Brunner, a businessman who had the support of the U.S. Chamber of Commerce, and Sarah Steelman, a former state treasurer backed by Sarah Palin.

Felicia Sonmez contributed to this report.