Appearing on MSNBC's "Morning Joe" program, the Democrat said Akin, the Republican, had been fairly nominated in early August and should stay on the Nov. 6 ballot.
"I really think that for the national party to try to come in here and dictate to the Republican primary voters that they're going to invalidate their decision, that would be pretty radical, and I think there could be a backlash," McCaskill said.
The furor over Akin's weekend comments -- that pregnancy from "legitimate rape" is rare -- grew Monday. Some Republicans said Akin should consider withdrawing from the campaign in favor of another candidate.
"If the rape comment had been the only problem, there might be an argument he could make to stay in the race," wrote Rick Moran on the conservative American Thinker website. "But since the primary, Akin has put his foot in his mouth several times, referring to student loans as 'socialism' and calling for an end to school lunch programs. ... Todd Akin should withdraw from the race."
There was no indication Monday morning that Akin was considering such an option. Akin has no scheduled campaign appearances Monday, his campaign said.
On Sunday Akin said he "misspoke" in the televised interview with a St. Louis television station in which the rape issue was discussed.
Under Missouri law, candidates can withdraw from a statewide race without penalty "not later than the eleventh Tuesday prior to the general election." That deadline is tomorrow.
Candidates can withdraw after the deadline with a court order. The party's state committee would pick the replacement candidate for statewide offices.
Republican presidential nominee Mitt Romney Monday denounced Akin's statement in an interview with National Review Online, a conservative website.
"Congressman's Akin comments on rape are insulting, inexcusable, and, frankly, wrong," Romney said. "Like millions of other Americans, we found them to be offensive."