By Kevin Murphy

KANSAS CITY, Missouri (Reuters) - Republican Todd Akin and incumbent Democrat Claire McCaskill debated Medicare, immigration and the national debt on Thursday, but did not discuss Akin's "legitimate rape" remark that drew national attention to their Missouri U.S. Senate race.

Akin's comment in August that women have biological defenses against pregnancies that might be caused by "legitimate rapes," which he later retracted, drew widespread rebukes even from members of his own party.

The six-term congressman from St. Louis resisted requests that he step aside for another candidate to take on McCaskill, who had been seen as one of the most vulnerable incumbents in the Republican effort to win a majority in the Senate.

McCaskill's campaign earlier this month released an ad featuring rape victims to highlight the Akin gaffe. Akin has faced an uphill battle for financial support since making the comments, although some conservative Republicans have renewed support for him since he stayed in the race.

Since the Akin comments, McCaskill has led most polls by single digits, but Republicans are not giving up hope because Missouri has trended more conservative in recent years.

Republicans need a net gain in the November 6 election of four seats in the 100-member U.S. Senate to take the majority from Democrats.

At the end of September, McCaskill had $2.1 million cash on hand, nearly quadruple the $553,000 Akin had, according to campaign finance reports filed with the Federal Election Commission this week.

Thursday's debate touched only briefly on abortion, with Akin stressing that he was opposed and McCaskill in favor of keeping abortion legal.

In closing, McCaskill said women in Akin's employ the past 12 years had earned an average of 23.4 percent less than men.

"He supports the boss being able to decide whether or not you get paid less simply because you are a woman," she said.

Akin did not respond directly to McCaskill's assertion, but has said he believes pay should be left to the discretion of employers. Akin said McCaskill must share the blame for the slumping economy and growing national debt.

"Claire McCaskill seems to want to escape from the economic mess that has been created in Washington," Akin said.

Akin said McCaskill voted 98 percent of the time with President Barack Obama, including support for his healthcare program and other issues. McCaskill said her voting record was moderate and many of those votes were procedural.

McCaskill raised $5.8 million in the three months ended September 30, compared with Akin's $1.6 million.

(Editing by David Bailey, Greg McCune and Peter Cooney)