According to Buzzfeed:

Akin is taking concrete steps to withdraw by tomorrow at 5:00 p.m.,” a senior Republican told BuzzFeed, adding that Akin could still change his mind.

Tomorrow afternoon is an important deadline: If he files papers to end his candidacy in Jefferson City tomorrow he can end his candidacy unilaterally; after that, he would have to file an application in court.

Possible Republican replacements, the source said, include three former senators: John Ashcroft, Jim Talent, and Kit Bond. Other names being mentioned include former Ambassador Ann Wagner, who is running for Akin’s now-vacant seat; and Auditor Tom Schweich.