Clinton, speaking at the University of Pittsburgh-Johnstown, asked voters to look past Murtha's recent controversial remarks that that some of his constituents are racists and rednecks.
"You are so lucky to be represented in Congress by this man and don't you let anything happen to him. Don't vote on resentments," Clinton said.
In an October interview, Murtha described western Pennsylvania as racist, apologized for the remark, then told another interviewer the area had been "really redneck" in years past.
Murtha also angered some veterans for his comments on the role the U.S. Marines played in the controversial 2005 shootings of Iraqi civilians in Haditha. Murtha said the troops "overreacted" and killed "innocent civilians in cold blood." One of the Marines exonerated in the killings has sued Murtha for slander.
At a rally for Murtha's opponent, Bill Russell, on Sunday, Shawn Bryan, a former sergeant in the U.S. Marine Corps., called Murtha a "fat little bastard" who betrayed Marines everywhere by those statements.
Speaking with FOXNews.com Monday, Bryan said he didn't regret his attack on Murtha, but he said he should have employed a better choice of words.
"He has betrayed the U.S. Marine Corps time and time again. I would hope that nobody out there would change their vote because of the derogatory words I used, but I want people to know the truth. It's about time somebody called a spade a spade," said Bryan.
Murtha, who spent 37 years in the Marine Corps and was awarded a Bronze Star and two Purple Hearts, faces a surprising challenge from Russell, a retired Army officer who moved to Johnstown to run against the Democratic incumbent.
Murtha has typically won his races by large double-digit margins, but now he's fighting for his political life.
Murtha has criticized Russell for supporting the privatization of Social Security, and he has emphasized his own record of bringing jobs and billions of dollars to the district. He has also touted his strength in procuring earmarks for his district, but Russell has said they make the area too dependent on federal money.
Russell said Monday that he didn't believe Clinton's visit would do anything to motivate voters who are upset with Murtha.
"I think it will make a bit of a difference in terms of shoring up those who are already supporting him, but I think there are going to be a lot of people showing up just to see Bill Clinton," Russell said before the Clinton rally. Russell planned to speak with reporters at an afternoon news conference at his headquarters in Johnstown.
The Associated Press contributed to this report.