Duckworth's campaign on Tuesday called the comments an insult to veterans, while Walsh insisted that his words were being distorted and that he indeed believed she was a war hero.
He said political advisers for McCain, the 2008 Republican presidential nominee, "day after day had to take him and almost throw him against a wall and hit him against the head and say, 'Senator, you have to let people know you served!'"
"That's what's so noble about our heroes," Walsh said. "Now I'm running against a woman who, I mean, my God, that's all she talks about. Our true heroes, the men and women who served us, it's the last thing in the world they talk about."
McCain wrote about his experiences in a best-seller, "Faith of My Fathers," and during his GOP acceptance speech in 2008, he referred to his time as a POW.
Walsh's remarks are being publicized by two groups that have begun online petitions against him.
The petition from VoteVets.org says it's time to teach Walsh that "all of our veterans are heroes and that we can never recognize them enough for their service." The petition from EMILY's List, which supports Democratic women who favor abortion rights, urges: "Stop the GOP from taking down our pro-choice champions. Tell Joe Walsh to stop questioning Tammy Duckworth's heroic service to our country!"
Duckworth, a former state and federal veterans affairs official from Hoffman Estates, is challenging Walsh in the Nov. 6 election for Illinois' 8th Congressional District seat in the north and northwest suburbs. She is a lieutenant colonel in the Illinois Army National Guard.
Kaitlin Fahey, Duckworth's campaign manager, said in a statement Tuesday: "Congressman Walsh's comments insult those who sacrificed to make this country free."
In response to the Democratic criticism, Walsh issued a statement calling the controversy a "political ploy to distort my words and distract voters."
"Of course Tammy Duckworth is a hero," he said. "I have called her a hero hundreds of times."
He added, though, that most veterans he has met since entering Congress rarely talk about their service or combat experiences while "that is darn near all of what Tammy Duckworth talks about."
McCain, too, has been a target of Walsh, who last spring said voters elected Barack Obama because he was an African-American historic figure and added: "It helped that John McCain was about 142 years old."