WASHINGTON -- The Wisconsin Democratic Party is out with its first ad in the campaign to recall Gov. Scott Walker (R), comparing the governor to former President Richard Nixon.

The ad centers on the John Doe investigation -- dubbed "Walkergate" by Democrats -- that has already resulted in charges against six of Walker's top aides. The probe by the Milwaukee County district attorney’s office is reportedly focusing on whether staffers who worked for Walker did political work with taxpayer money. Walker previously served as Milwaukee County executive.

The ad compares Walkergate to Watergate, putting images of Walker next to footage of Nixon and the infamous corruption scandal of the 1970s.

The spot ends with former Sen. Howard Baker saying, "What did the president know and when did he first know it?" and MSNBC host Ed Schultz also saying, "What did the Scott Walker know and when did he know it?"

(Watch the ad above.)

In an email to supporters announcing the new ad, which is expected to start airing Monday night, Wisconsin Democratic Party Chair Mike Tate writes, "The ad is called 'Walkergate' because it highlights the eery parallels between Nixon's Watergate scandal and the criminal investigation into Scott Walker's current and former top-level aides."

"Much of the illegal activity took place less than ten yards from Scott Walker's desk, but instead of coming clean about what he knew and when he knew it, Walker has been meeting with big shot Chicago attorneys while David Koch 'helps him' by flooding Wisconsin with propaganda ads that divert attention away from the growing scandal," he adds.

During a conference call Monday morning with reporters, Tate was asked why the ad focuses on the John Doe probe rather than the issue of collective bargaining, which first sparked labor and progressive anger at Walker last year.

"The reason we're running this ad is because we believe people -- particularly in northeastern Wisconsin and central and western Wisconsin -- don't know as much about the John Doe probe as do voters in the rest of the state," he said.

A recent poll by Marquette University Law School found that Walker's support has dropped in the past month, and the John Doe investigation may be a key factor. Walker has a 46 percent favorable rating, compared to 50 percent last month, according to the poll. Fifty-two percent of respondents aware of the probe said it was "really something serious," while 40 percent said it was "just more politics."

Tim Russell, who once served as Walker's campaign aide, deputy chief of staff and county housing director, is "accused of embezzling more than $20,000 in money meant for veterans and using it for personal expenses," according to the Wisconsin State Journal. Other charges in the investigation "allege a pattern of illegal fundraising and what appears to be a systemic avoidance of campaign laws by Walker's inner circle."

Walker has said he is "voluntarily" cooperating with the Milwaukee investigation. On MSNBC Monday morning, he called the recall effort a "distraction."

“It's $9 million of taxpayer money just to run this," he said on MSNBC's "Morning Joe." "Think about the number of kids, think of the number of seniors we could help in our state with $9 million we didn't have to waste on this frivolous recall election."

Last month, Democrats turned in more than one million petition signatures in the effort to trigger a recall election of Walker -- far more than the number required. The signatures are still being verified and challenged by Republicans, but both sides are already preparing for a possible recall election.

UPDATE: 2:00 p.m. -- Tom Evenson, press secretary for the Friends of Scott Walker Campaign, emailed the following statement in response to the ad.

The Democrat Party and the big-government union bosses funding them are using lies and distortions in their negative attack ad to discredit Governor Walker in an attempt to take Wisconsin back to the failed days of billion-dollar budget deficits, double-digit tax increases, and record job loss. The truth is, Governor Walker 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 to deflect from the fact that Governor Walker’s reforms have laid the foundation for a more successful Wisconsin.