Abortion became a more prominent issue in the presidential campaign when Romney told the Des Moines Register, "There's no legislation with regards to abortion that I'm familiar with that would become part of my agenda." The campaign then began airing a TV ad in which a woman touts Romney's support for more moderate legislation.
But the Obama campaign hit right back, releasing a spot of its own that features footage from a 2007 GOP presidential primary debate in which Romney said he would be "delighted" to sign a bill banning all abortions in the U.S.
Mourdock is the third Republican Senate candidate, all of them men, who has made waves with his views on rape and abortion. In August, Rep. Todd Akin said during an interview in August that "the female body has ways to try to shut that whole thing down" and block pregnancy in cases of what he called "legitimate rape." Akin apologized for the comment, but refused to leave the race despite pressure from his own party.
Later in the month, Pennsylvania's Tom Smith likened his own daughter's out-of-wedlock pregnancy to conception through rape.
Smith quickly backed off the statement when pressed by reporters about whether he meant to conflate the two.
"No, no, no," Smith said, before seeming to qualify his defense: "Put yourself in a father's position," he said. "Yes, I mean it is similar."
"It's life," Smith explained, eventually, "and I'm pro-life. It's that simple."
Both Akin and Smith are behind in their races to unseat incumbent Democrats Sen. Claire McCaskill and Sen. Bob Casey, respectively. Mourdock had a five-point lead on Donnelly in the most recent Rasmussen poll. An internal survey commissioned by Donnelly showed the Democrat leading Mourdock, 40-38.
ABC News' Emily Friedman and Shushannah Walshe contributed to this report