As they take to the trail this Labor Day for the traditional opening of the fall campaigns, Republican and Democratic candidates say the Lewinsky matter, a jittery stock market and tumult abroad have thrust them into the most unsettled, and unpredictable, election season in years.

The uncertainty has already altered the calculations of Republicans, who say they have the most to gain, particularly from President Clinton's difficulties.Newly emboldened, Republican officials have revised their earlier predictions of modest gains and now say the party could capture more than a dozen additional seats in the House and, perhaps, even pick up five seats in the Senate. That would hand Republicans the magic 60 votes needed to cut off a filibuster when Democrats try to block a vote.

By contrast, Democrats find themselves on the defensive, and party leaders are lowering their expectations; most have stopped brazenly forecasting that they will retake the House.

"I feel so much better than a couple months ago," said Brent Siegrist, the Republican leader of the Iowa House. "I do think Clinton's troubles help us."

Although Republicans now have the slimmest House majority held by any party in the past half century, few Democratic leaders are upbeat about their party's prospects for retaking the House, and some party candidates are distancing themselves from Clinton.

"I'm running my own race - and I don't plan on running any events with him," said Ken Lucas, a Democrat who is locked in a tight House race in Kentucky. The latest Democrat to spurn Clinton is Maryland Gov. Parris Glendening, who said he "will not pursue the attendance of the president" as planned for a fund-raising event next month.

Here in Boston, at the annual meeting of the American Political Science Association, the corridor chatter among academics was not about political theory but fresh gossip about the Lewinsky investigation and how it would play out on Election Day. Perhaps a telling piece of evidence was that among the hundreds of academic papers presented, the association ran out of copies for one in particular demand: "Compassion, Political Trust and Public Opinion."

Rep. Jim McDermott, D-Wash., said the "formless uncertainty" in the electorate may be welcome news for Republicans trying to maintain their 11-seat majority in the House. He suggested that voters may want to keep GOP control of Congress as a check on the turbulence in the White House.

"In many ways, what people want is stability in our government, which I think bodes well for incumbents," McDermott said. "I wish they wanted to throw out the Republican majority and bring in Democrats, but I think the uncertainty fosters the stability."

Voter participation in the statewide primaries this year has been the lowest on record, which suggests that turnout in November could be the lowest in history, according to a study by Curtis Gans, director of the Committee for the Study of the American Electorate. He and other experts said that could bode well for Republicans.

Their hope is that while Democrats will be uninterested, Republicans will be energized by Clinton's troubles and will eagerly head to the polls to vote against his party.

"It doesn't look like '94, where we had a bump up out of anger," Gans said. "What you have at this point is disengagement but not profound anger - except on the Republican right." If conservative voters are motivated by their anger at Clinton, he said, that would give Republicans an edge in close contests.

Yet for all the theories about depressed Democratic turnout and an excited Republican electorate, polls thus far have not shown any major upheaval in how people expect to vote or broad disaffection with the Democratic Party. Despite signs that Clinton's personal approval is eroding, his job approval rating remains impressive. And despite Democrats' complaints that the Lewinsky debacle is crowding out their attempts to talk about issues, there is no statistical evidence that Clinton is dragging down Democratic candidates.

In fact, much of the sense of a changed electoral landscape in both the Republican and Democratic parties is founded on anecdotal evidence. Two weeks ago, Stuart Rothenberg, an independent political analyst, predicted that the Democrats would pick up two or three seats in the House. Now, he says, the Republicans could capture 10 or 15 seats.

Asked to explain his change of heart, Rothenberg said: "I don't have micro or macro data that shows either a national move or a move in individual races. We're going on gut here. You don't discount gut. But you don't go too far."