A lobbyist with ties to Republican candidate Dino Rossi is suing supporters of Democratic Gov. Christine Gregoire for defamation because he was mentioned in a political-attack ad.

OLYMPIA — A lobbyist with ties to Republican candidate Dino Rossi is suing supporters of Democratic Gov. Christine Gregoire for defamation because he was mentioned in a political-attack ad.

Lobbyist David Ducharme’s lawsuit claims the independent political group Evergreen Progress is harming his reputation, because the ad in question appears to tie him to a campaign-financing lawsuit involving the Building Industry Association of Washington, or BIAW.

Ducharme’s lawsuit was filed Monday in King County Superior Court. He’s asking a judge to order the ad pulled off the air. He also wants unspecified damages.

The ad in question opens with images of Rossi, Ducharme and Ducharme’s father, Richard Ducharme. It says “these real-estate industry lobbyists” helped Rossi buy an apartment building and a bank.

The ad then goes on to say that “Rossi’s big developer friends” are now embroiled in a campaign-financing controversy, including a lawsuit from Republican state Attorney General Rob McKenna — a reference to the BIAW’s recent court case alleging misreported campaign financing.

David Ducharme’s lawsuit acknowledges that he has invested in the two properties with Rossi but disputes the characterization that he “helped” Rossi buy them.

The younger Ducharme also says the ad falsely asserts that he’s a “real-estate industry lobbyist,” and falsely ties him to the BIAW fundraising lawsuit.

David Ducharme says he doesn’t work for the BIAW, and is not involved with its campaign financing. Ducharme says he did lobby for a group of employers involved in workers’ compensation programs with the state, and the association included BIAW. But Ducharme says that was in 2000.

Evergreen Progress is bankrolled mainly by the national Democratic Governors Association, and labor unions. It’s an independent spending committee supporting Gregoire and opposing Rossi. The BIAW is financing its own third-party effort to support Rossi and oppose Gregoire.

An Evergreen Progress official, Jason Bennett, had no immediate comment on the lawsuit Thursday. The group’s chairman, Rick Desimone, did not immediately return a call seeking comment.

Ducharme’s lawsuit is the latest courtroom twist in the governor’s race, a heated rematch between Rossi and Gregoire. Gregoire won the governor’s mansion in 2004 by just 133 votes, after two recounts and a failed Republican court challenge. Polls show the two candidates running very close.

The BIAW is embroiled in two lawsuits alleging that the conservative business group broke state campaign-financing laws while raising money to support Rossi’s 2008 run.

The first lawsuit, over alleged missteps in reporting the sources of some campaign money, is being pursued by Republican state Attorney General Rob McKenna. It’s directly mentioned in the Evergreen Progress ad.

The second lawsuit is from two former Supreme Court justices — both Gregoire donors — who allege that the BIAW illegally coordinated its fundraising with Rossi. They want to make Rossi testify under oath in the case before the Nov. 4 election.

Ballots are already on their way to voters in Washington’s largely vote-by-mail election.