The Wisconsin Democratic party is releasing its first ad in the recall campaign against Gov. Scott Walker (R) Monday.

“Walkergate” compares a John Doe investigation into current and former Walker aides to the Watergate scandal, juxtaposing news clips about the probe with coverage of the 1970s Nixon scandal.

Petition signatures to recall Walker were filed earlier this year, in response to legislation that eliminated collective bargaining rights for many public employees. Those signatures have yet to be certified, but a recall election against the governor is likely to take place later this year.

As that fight goes forward, prosecutors are investigating four aides and appointees of Walker’s from when he was Milwaukee County executive. Two Walker appointees have been charged with embezzling funds intended for veterans; two aides were charged with illegal fund-raising.

“There are many similarities by Watergate and Walkergate,” said Wisconsin Democratic Party Chairman Mike Tate on a conference call with reporters, saying Walker “has given a blizzard of contradictory statements about what he knew and when he knew it.”

Walker says he is not a target of the probe, but he has hired two defense attorneys and is meeting with prosecutors.

Former Dane County Executive Kathleen Falk and state Sen. Kathleen Vinehout have already entered the Democratic primary to take on Walker.

Democrats would not disclose the amount of money being spent on the “Walkergate” ad, saying only that it’s a rolling buy in multiple markets.

Walker Press Secretary Tom Evenson said the ad was full of “lies and distortions” and that the governor “immediately addressed any issues of misconduct when brought to his attention. ... The character assassination being conducted by Madison Democrats and big-government union bosses in this ad shows they are grasping at straws.”

Walker has been airing ads since the recall petition drive kicked off last fall; he has already raised and spent millions. The political action committee Americans for Prosperity, funded by the billionaire libertarian Koch brothers, has already put hundreds of thousands of dollars into ads supporting the governor.

A recent poll showed Walker leading all likely challengers.