The final deadline for candidates to remove their names from the Missouri ballot passed Tuesday as Akin began a statewide bus tour. Akin is trying to unseat Democratic Sen. Claire McCaskill.
Akin has repeatedly apologized since saying during a television interview in August that women's bodies have ways of averting pregnancy in cases of what he called "legitimate rape."
Top Republicans, including presidential nominee Mitt Romney, have urged him to drop out. He's also lost backing from the national GOP.
But he has the state party's support, and former U.S. House Speaker Newt Gingrich joined Akin at a fundraiser Monday.