Twelve years ago, during George W. Bush's presidency, Republican scandals turned a bad mid-term election into a terrible one. Unexpectedly, Democrats recaptured House and Senate majorities.
This year, under President Trump, Republicans suffer from more and deeper scandals. Could Democrats exploit them for the same November effect?
This week's arrest of GOP Rep. Chris Collins, charged with conducting insider trading while attending a White House picnic, amplified a question looming over election season. But scandal represents an uncertain tool for political campaigns - which sometimes can even backfire.
In 2006, allegations of financial misbehavior rocked a series of GOP members. House Majority Leader Tom DeLay quit under a cloud; two colleagues, Duke Cunningham and Bob Ney, eventually went to jail.
More salacious: reports surfaced that Rep. Mark Foley had sent sexually-suggestive messages to male teenaged pages in the Capitol. The Foley news hit in September at an especially vulnerable moment for Republicans.
But their greatest vulnerabilities then stemmed from the foundering Iraq War and the Bush administration's botched response to Hurricane Katrina. GOP scandals gave Democrats a boost that was "meaningful," pollster Mark Mellman says, "but not decisive."
Today, the scandal trail begins at the White House. Trump faces a lawsuit accusing him of profiting illegally from his office, allegations of sexual misconduct and hush money, and the Russia investigation by Special Counsel Robert Mueller.
Trump's campaign chair is standing trial on bank and tax fraud charges. His former National Security Adviser and deputy campaign chief have plead guilty to felonies.
Three Trump Cabinet members – Tom Price, Scott Pruitt and David Shulkin – quit under ethical clouds. At least four others - Steven Mnuchin, Ryan Zinke, Ben Carson and Wilbur Ross - have faced scrutiny from internal watchdogs over use of taxpayer resources, involvement of family members in government business, and failure to comply with a divestment directive.
Two White House aides resigned over allegations of domestic violence. A ProPublica investigation recently found that three members of Trump's Mar-A-Lago club have exerted substantial influence over the Veterans Administration from outside of government.
On Capitol Hill, Rep. Collins has company in trouble. Freedom Caucus firebrand Jim Jordan faces allegations that he ignored sexual abuse of wrestlers he coached at Ohio State University. Two other members, Trent Franks and Tim Murphy, quit in personal disgrace.
As long as that list is, something larger dominates this election season, too: Trump's uniquely polarizing governing style. Allegations of ethical misconduct may contribute to his historic unpopularity, but didn't cause it.
In the campaign's home stretch, Democratic candidates must decide how much to emphasize what House Minority Leader Nancy Pelosi calls the "culture of corruption." Many prefer to talk about issues closer to voters' lives, like the GOP tax-cut benefiting the wealthy more than the middle-class, and attacks on Obamacare that are fueling health insurance premium increases.
Cynicism about the ethics of both parties should inform that choice. In a May Pew Research survey, Americans' assessments of Republican and Democratic ethical standards were essentially identical.
Yet Democratic pollster Anna Greenberg insists that scandals bolster her party's ongoing argument that Republicans do the bidding of moneyed interests. When Democracy Corps recently asked Americans to name each party's top priority, the most common response about Democrats was "health care"; about Republicans, was "themselves."
Nor do Republicans dismiss the issue. Pollster Neil Newhouse said concerns over ethical misconduct may prevent some voters, who are otherwise happy with the state of the economy, from returning home to the GOP.
Neither party has forgotten what happened in 1998, when Republicans made a late attempt to capitalize on President Bill Clinton's affair with Monica Lewinsky – and ended up losing House seats. No one outside of Mueller's investigation can say if or when he may publicly offer new allegations even closer and more damaging to the president.
"The thing we don't know yet is what the other shoe is, or even if there is one," concluded Republican strategist Chris Wilson. "That's probably the difference between a regular bad year – potentially losing the House – and a real wipe-out."
—By CNBC's John Harwood. Follow him on Twitter: @JohnJHarwood