WASHINGTON (Reuters) - A spate of sexual misconduct accusations against U.S. politicians and other powerful men will force candidates for the November 2018 congressional elections to weigh more carefully than ever whether their past behavior could doom their chances.

Following allegations against Republican U.S. Senate candidate Roy Moore, Democratic U.S. Representative John Conyers and Democratic U.S. Senator Al Franken, campaign operatives from both parties warned that past behavior that might once have been excused may now be disqualifying.

“This is a game-changer,” said Democratic strategist Dane Strother. “Every man who wants to run for office needs to give some serious thought to his past.”

Politicians have been among a growing number of prominent men, including in the entertainment and media fields, accused of sexual harassment.

There will be increased pressure on candidates to undertake “self-vetting,” where, as one Republican strategist said, they are willing to subject themselves to a “trial on what the other side will put them through.”

But he cautioned: “A lot of this is still dependent on what the candidate is willing to talk about and how forthcoming they are.”

In next year’s elections, Democrats will seek to wrest one or both houses of Congress from Republican control. Thirty-three U.S. Senate seats and all 435 seats in the U.S. House of Representatives will be contested.

Sex scandals have long been a part of U.S. politics, but in the current environment, operatives are encouraging candidates and office-holders alike to level with advisers about past conduct, even behavior that might in the past have fallen into a gray area.

“You may have to press the candidate particularly aggressively to be sure that he confronts what he may have passed off as a failed advance and not have imagined would come back to haunt him,” a veteran Democratic lawyer who advises campaigns told Reuters.

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GOING ‘OPPO’

Sonia Van Meter, a Democratic opposition researcher, said candidates would have to think carefully about “their demeanor, their offhanded remarks, the way they carried themselves. Everything will be under more scrutiny.”

If candidates are not careful about self-vetting, operatives said, researchers working for opposing candidates would do it for them.

Verifiable facts - court documents, voting records, speeches and more - usually form the backbone of opposition research conducted by rival campaigns.

Such research may expand into behavior that has not been documented, Strother said, adding that might include conversations with former female staffers to find out if there are any issues.

Tracy Sefl, a strategist in Chicago who has directed opposition research for the Democratic National Committee, said workplace relationships between a male candidate and women could now be targeted by opposing campaigns.

“In the context of employment: Was a man supervising women? Were those women younger, older, or his peers?” Sefl said. “How long did those women tend to work there? What do they have to say about their experience there and about him, specifically?”

But Alex Conant, a Republican consultant who worked for presidential candidates Tim Pawlenty and Marco Rubio, doubted opposition research would see a dramatic shift, largely because most campaigns cannot afford it and must instead rely on public records, internet searches and news reports.

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“Opposition research in early stages of congressional campaigns is not going back and interviewing every single person who has worked with a candidate,” Conant said.

There are also limits on the effectiveness of opposition research in identifying potential misconduct.

Rumors surrounding Moore’s alleged interest in teenage girls had circulated in Alabama politics for years, but it took reporters from the Washington Post, not researchers from campaigns, to persuade his accusers to go on the record.

Moore, who is running against Democrat Doug Jones in a Dec. 12 special election, has denied the allegations, which Reuters has been unable to independently verify. Republican lawmakers in Washington, including Senate Republican leader Mitch McConnell, have distanced themselves from Moore and urged him to quit the race.

ALTERED LANDSCAPE

Self-vetting is all the more important, operatives said, because the national parties have little capacity to weed out problematic candidates.

In 2016, the insurgent White House bids of Republican Donald Trump and Democrat Bernie Sanders highlighted the limited ability of parties to hand-pick candidates for major public offices.

Trump won the presidency despite himself being accused by several women of having in the past made unwanted sexual advances or inappropriate personal remarks about them. Trump denied the allegations, saying they were part of a smear campaign.

In the race for the U.S. Senate seat in Alabama, Moore prevailed in the Republican primary over Luther Strange, the incumbent backed by the party establishment.

While the parties still vet some candidates for Congress and big-ticket donors, they largely rely on local party officials to screen and refer them.

Van Meter said the string of scandals had altered the landscape because victims now felt empowered to go public, which may topple some candidates and keep others from running at all.

“What has changed is the culture, the number of women coming forward,” she said. “We’re going to see more of these cases.”