Missouri Republican Senate candidate Todd Akin resisted calls to withdraw from the race Monday over his controversial comments on rape, despite mounting pressure from GOP leaders who roundly condemned his remarks and threatened to cut off funding.

"I am in this race to win. We need a conservative Senate," Akin tweeted Monday afternoon, as he solicited new donations. He also told Fox News' Sean Hannity on his radio show that "we're going to stay in."

The tweet followed scattered and unconfirmed reports that Akin was moving to withdraw from the race. Akin, though, rejected those claims publicly even as Republican leaders leaned on him hard to reconsider his bid for Senate.

A source within the National Republican Senatorial Committee told Fox News that Sen. John Cornyn, head of the Senate GOP campaign arm, already has told Akin that if he stays in the race, the $5 million set aside for the Missouri race will be withdrawn. Cornyn, according to the source, told him the party is concerned his presence in the race could imperil Republicans' chances of winning the Senate majority.

Akin, a six-term GOP congressman, is challenging Democratic Missouri Sen. Claire McCaskill for her seat. His chances looked fairly sunny -- up until he told an interviewer with Fox affiliate KTVI that a woman's body can typically prevent pregnancy during a "legitimate rape," as he argued against allowing abortions in cases of rape, claiming such pregnancies are uncommon in the first place.

"It seems to me first of all, from what I understand from doctors, that's really rare," Akin told KTVI. "If it's a legitimate rape, the female body has ways to try to shut that whole thing down."

Akin has since backed off the comment, saying he "misspoke." Akin apologized for the remark Monday.

Adding to the wave of condemnation, Senate Republican Leader Mitch McConnell called his comments "totally inexcusable."

"What he said is just flat wrong in addition to being wildly offensive to any victim of sexual abuse," McConnell said. "Although Representative Akin has apologized, I believe he should take time with his family to consider whether this statement will prevent him from effectively representing our party in this critical election."

Several other GOP Senate candidates were also putting serious distance between their campaigns and his, as they tried to head off Democratic efforts to link Akin's comments to other members of the party.

GOP Sen. Scott Brown, who's in a tough race against President Obama ally Elizabeth Warren in Massachusetts, tweeted that he found Akin's remarks to be "outrageous."

He followed up with a written statement calling on Akin to bow out.

"As a husband and father of two young women, I found Todd Akin's comments about women and rape outrageous, inappropriate and wrong," Brown said. "There is no place in our public discourse for this type of offensive thinking. Not only should he apologize, but I believe Rep. Akin's statement was so far out of bounds that he should resign the nomination for U.S. Senate in Missouri."

Further, NRSC Chairman Cornyn used a brief written statement to nudge Akin to reconsider his bid for Senate.

"Congressman Akin's statements were wrong, offensive, and indefensible," Cornyn, R-Texas, said. "I recognize that this is a difficult time for him, but over the next 24 hours, Congressman Akin should carefully consider what is best for him, his family, the Republican Party and the values that he cares about and has fought for throughout his career in public service."

Obama, speaking in the White House briefing room, also called the comments "offensive." "Rape is rape," Obama said, and the idea of distinguishing among types of rape "doesn't make sense to the American people and certainly doesn't make sense to me."

Aside from Brown, other Republicans in tough Senate races were similarly critical, without calling on Akin to step aside.

Rep. Jeff Flake, who is running for Senate in Arizona, tweeted that Akin's comment was "wrong."

"I oppose abortion, but exceptions must be made for rape, incest and to protect the life of the mother," he said.

Virginia Senate candidate George Allen and Montana Senate candidate Rep. Denny Rehberg reportedly have joined in the rebuke -- as has Akin's former primary opponent Sarah Steelman, who tweeted that she found the comments "inexcusable, insulting and embarrassing to the GOP."

Mitt Romney gave a similar verdict in an interview with The National Review.

"Congressman Akin's comments on rape are insulting, inexcusable, and, frankly, wrong," Romney said.

Nevertheless, the Akin remarks became fast fodder for the Democratic National Committee, as Chairwoman Debbie Wasserman Schultz tried to link Akin's stance to the Romney campaign.

"Now, Akin's choice of words isn't the real issue here. The real issue is a Republican Party -- led by Mitt Romney and Paul Ryan -- whose policies on women and their health are dangerously wrong," she said in an email directing supporters to sign an anti-Romney petition on the DNC website.

She cited Romney's past comments on wanting to "get rid" of federal funding for Planned Parenthood.

The DNC email follows a pattern of trying to characterize the Romney ticket as "dangerous" to women, fueled in this case by comments from a congressman not tied to the presidential campaign.

McCaskill, who is seeking a second term, said in an emailed statement Sunday that she found the comments "offensive."

"It is beyond comprehension that someone can be so ignorant about the emotional and physical trauma brought on by rape," McCaskill said. "The ideas that Todd Akin has expressed about the serious crime of rape and the impact on its victims are offensive."

Akin clarified his remarks in a written statement.

"In reviewing my off-the-cuff remarks, it's clear that I misspoke in this interview and it does not reflect the deep empathy I hold for the thousands of women who are raped and abused every year," Akin's statement said. "Those who perpetrate these crimes are the lowest of the low in our society and their victims will have no stronger advocate in the Senate to help ensure they have the justice they deserve."

Akin also said in the statement he believes "deeply in the protection of all life and I do not believe that harming another innocent victim is the right course of action."

Fox News' John Brandt and The Associated Press contributed to this report.