Two other Republican senators, Scott P. Brown of Massachusetts and Ron Johnson of Wisconsin, urged Mr. Akin to resign.
Mr. Akin also apparently has lost a key source of funding. Sen. John Cornyn, head of the National Republican Senatorial Campaign Committee, told Mr. Akin that $5 million in advertising set aside for Missouri will be spent elsewhere and that Mr. Akin will get no other help from the committee, according to a committee official who spoke on condition of anonymity because the conversation was private.
Republican frustration grew Tuesday. Two GOP officials, speaking on the condition of anonymity because they did not want to irritate Mr. Akin, said party officials seeking to talk with him were having trouble reaching him Monday night and Tuesday morning.
Akin campaign spokesman Ryan Hite declined Tuesday to reveal Mr. Akin‘s whereabouts but said he was not in his suburban St. Louis campaign office. Mr. Hite said the campaign may release information about his public schedule later.
At least one political interest group that has pounded Mrs. McCaskill with attack ads, the Karl Rove-backed American Crossroads, also pulled its ads from Missouri.
The apology video Mr. Akin posted on YouTube early Tuesday was an apparent attempt to claw back some of that funding.
“Fact is, rape can 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 said in the video.
President Obama said Monday that Mr. Akin’s comments underscore why politicians — most of whom are men — should not make health decisions on behalf of women.
“Rape is rape,” Mr. 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.”
It was just two weeks ago that Mr. Akin was at the top of the political world in Missouri after winning a hotly contested three-way battle with millionaire businessman John Brunner and former state Treasurer Sarah Steelman for the right to challenge Mrs. McCaskill in the November election. Missouri has grown increasingly conservative in recent years, and Mrs. McCaskill is seen as vulnerable.
She was not among those calling for her opponent to get out of the race.
“What’s startling to me is that (Republican) Party bigwigs are coming down on him and saying that he needs to kick sand in the face of all the primary voters,” Mrs. McCaskill said at a campaign event Monday in suburban St. Louis. “I want Missourians to make a choice in this election based on policy, not backroom politics.”
One anti-abortion group expressed support for Mr. Akin, while another called on him to step aside.
Missouri Right to Life, which opposes a woman’s right to get an abortion even in cases of rape and incest, said Mr. Akin‘s “consistent defense of innocent unborn human life clearly contrasts” with Mrs. McCaskill’s position.