Their battle is playing out in Illinois' 8th Congressional District.
In a video, Congressman Walsh encourages employers to say this to their workers: "If Obama is re-elected, if the Democrats take Congress, I may not be able to cover your health insurance next year," he said.
Walsh was referencing a growing number of Republican business owners who have notified employees in no uncertain terms that a vote for the president is a vote against their job security.
"If there's ever a year where people who run, manage, and own companies are going to energize their employees, it better be this year," Walsh said in the video.
On Thursday, Duckworth called the comments "irresponsible."
"Any time a United States Congressman encourages employers to threaten employees with their jobs if they don't vote the way their employer wants, that's shameful," said Duckworth.
"There should be obviously no pressure, no forced coercion on how to vote for anybody. This is simply information," said Walsh.
The video was shot and distributed by a left-wing Superpac and it underscores the influence of outside money in the race.
"This is one of the nastiest congressional races in the country. You have two very different candidates that both have national parties behind them," said Laura Washington/ABC 7 political analyst.
Duckworth has accused Walsh of violating election rules after a group that he co-founded helped to fund a Superpac now running ads against her.
"It is clear that Mr. Walsh has pledged his allegiance to some pretty large corporate interests and folks who are shadowy and will not show themselves to be who they are," said Duckworth.
Walsh dismissed the claim Thursday, saying he severed his ties to the group years ago and said Duckworth's use of outside money far outpaces his.
"She's not in the district. She's not campaigning publicly much. Most voters I think don't like that," said Walsh.