The Republican political establishment drew closer to a confrontation with some in the party’s Christian conservative wing Tuesday as Rep. Todd Akin of Missouri chose defiance over surrender, refusing to step aside as the GOP nominee for Senate.

Akin, whose controversial comments Sunday about “legitimate rape” and pregnancy set off a firestorm inside the party, said he intends to rally conservatives to a campaign focused on abortion, an issue he said was being ignored by the leadership of both major parties.

The escalation came as GOP leaders met in Tampa ahead of next week’s presidential nominating convention. They adopted a broad antiabortion position that was silent on whether exceptions should be allowed in cases of rape or incest, the issues that set off the Akin controversy.

Missouri’s Republican Senate primary already served to pit several conservative constituencies against one another, as Christian evangelical leaders backed Akin and hard-line anti-spending conservatives supported his opponents.

Mitt Romney, the GOP’s presidential standard-bearer, joined a broad chorus of Republicans urging Akin to step aside for the good of his party. “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,” Romney said.

But after two days of apologizing, Akin grew angry Tuesday, allowing a deadline to pass on an easier way to withdraw from the contest. The congressman made clear that he would not apologize for his belief that abortion should be illegal, even in cases of rape.

“I misspoke one word in one sentence in one day,” he said on a radio talk show hosted by former Arkansas governor Mike Huckabee. “I haven’t done anything that’s morally and ethically wrong.”

Tuesday evening, Akin tweeted: “I am #stillstanding. Will you stand with me?” He included a link to an online donation site.

The controversy began Sunday when a St. Louis television station aired an interview in which Akin was asked about his opposition to abortion, even if a woman becomes pregnant after being raped. “If it’s a legitimate rape, the female body has ways to try to shut that whole thing down,” he responded, adding that even if the woman became pregnant, “the punishment ought to be of the rapist and not attacking the child.”

The reaction from the Republican establishment was swift, and by Tuesday calls for Akin to step aside had increased from a trickle to a deluge.

Immediately after his appearance on Huckabee’s show, party leaders who had been sending Akin signals to quit the race left no doubt about where they stood.

“When the future of our country is at stake, sorry is not sufficient. To continue serving his country in the honorable way he has served throughout his career, it is time for Congressman Akin to step aside,” said Senate Minority Leader Mitch McConnell (Ky.).

A few hours later, Romney issued his statement calling on Akin to drop out. He was followed by Sen. John Cornyn, chairman of the National Republican Senatorial Committee, who reaffirmed plans to abandon a $5 million campaign for Akin. “If he continues with this misguided campaign, it will be without the support and resources of the NRSC,” said Brian Walsh, an NRSC spokesman.

The internal GOP debate over Akin has buoyed the hopes of Democrats, who acknowledged that Sen. Claire McCaskill (Mo.) was probably the most endangered Democratic incumbent seeking reelection this year. A McCaskill victory would present a much steeper hill for Republicans hoping to gain four seats to take majority control of the Senate.

And some GOP insiders worry that an Akin insurgency campaign could become a rallying point for antiabortion forces and a high-profile subject of division within the party’s base, maybe as soon as next week’s Republican National Convention, which is supposed to be a time of unity.

Economic vs. social issues

Party leaders had hoped that the Akin matter would be resolved well in advance of the meeting in Tampa so that the abortion issue would not be a sideshow to Romney’s coronation. Throughout the spring and early summer, when the campaign focused on President Obama’s stewardship of the economy, Romney advanced in the polls. But back in the winter, when the GOP primary tilted toward social issues, Obama edged ahead of his rivals.

This week, as more than 100 party officials gathered before the convention to map out the Republican platform, abortion took center stage. The platform committee adopted a proposal Tuesday calling for a constitutional amendment protecting “human life” — a broad antiabortion stance that says nothing about whether exceptions should be allowed in cases of rape and incest. Democrats labeled it the “Akin plank.”

The staunchest opponents of abortion, led by the Family Research Council’s Tony Perkins and the Eagle Forum’s Phyllis Schlafly, said they remained behind Akin’s candidacy Tuesday.

“Todd Akin . . . has a record of voting to protect human life,” Marjorie Dannenfelser, president of the Susan B. Anthony List, said in a statement. He “has been an excellent partner in the fight for the unborn.”

Akin’s defiance of Republican leaders in Washington and Missouri is the latest test of the clout of the GOP’s once-powerful establishment, which for more than two years has jousted with its base of conservative activists. To some extent, the powerlessness reflects the establishment’s failure in a string of 2010 Senate primaries to secure nominations for candidates it backed. That led to several embarrassing losses in the general election and a decision this year to stay out of contested primaries.

With Tuesday’s deadline passed, Akin now has until Sept. 25, should he reverse course, to petition the courts to remove his name from the ballot and replace it with that of another Republican, who would be selected by party leaders.

Cutting off campaign cash

Drying up Akin’s financial support may be the last card GOP leaders have left to play. He raised a little more than $2 million before the Aug. 7 primary, most of which was spent in the three-way race, from which he emerged with a plurality victory of 36 percent.

Besides the NRSC pulling its financial support, McConnell and Cornyn have cancelled plans to fete Akin at a Sept. 19 fundraiser in Washington with roughly a dozen GOP senators on hand, according to a senior NRSC aide.

It’s unclear that such political pressure can work on Akin, a six-term congressman from the St. Louis suburbs. In his House career, he has never been particularly close to party leaders such as Speaker John A. Boehner (Ohio) or Majority Leader Eric Cantor (Va.). He serves on the Armed Services Committee and is a proud advocate of earmarking funds for local projects, despite GOP leaders’ opposition to the now-banned practice.

As a member of the Budget Committee, chaired by the party’s vice-presidential pick, Rep. Paul Ryan (Wis.), Akin has regularly voted with a rump caucus of conservatives who think Ryan’s austere budget would not go far enough in cutting government spending.

His campaign team does not consist of seasoned GOP operatives and is instead run by his son.

Sen. Roy Blunt, the most powerful Republican in Missouri, issued a statement along with three former GOP senators calling for Akin to stand down his candidacy.

Instead, Akin launched an ad campaign that is partly an apology but mostly a defense of his antiabortion views. “The mistake I made was in the words I said, not in the heart I hold. I ask for your forgiveness,” he says in the ad.

Rosalind S. Helderman in Tampa contributed to this report.