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Michael P. McDonald, a professor at George Mason University who studies voting trends, said he expects 41.8 percent turnout among the voting-eligible population, which would translate to more than 90 million votes — a record for a midterm election.

Mr. McDonald also said nearly one in four voters will have cast a ballot either by mail or in person before Tuesday.

In Washington and Oregon, the entire vote is conducted early or by mail, while in Nevada and Colorado nearly two-thirds of the votes will have been cast early, Mr. McDonald projects.

“If Harry Reid survives in Nevada, it will be because of early voting,” he said.

Mr. Reid, Senate Democrats’ leader, campaigned Monday with first lady Michelle Obama, facing a strong challenge from conservative Republican Sharron Angle in one of the fall’s marquee matchups.

“I’m not finished fighting the big banks. They know it. I’m not finished fighting the insurance companies. They know it. I’m not finished fighting the oil companies,” Mr. Reid said.

That list of unfinished business underscored another worry for Democrats in Gallup’s polling. Its surveys found that despite passing reams of legislation this year, including overhauling the health insurance system and rewriting financial regulations, the majority party isn’t getting credit from voters for a robust session.

Less than a quarter of voters — 23 percent — said Congress has done more than usual this year, while 37 percent said lawmakers have accomplished less than normal.

If Republicans do win the House but not the Senate, it will be the first time since World War II that the lower chamber has flipped without the upper chamber.

Mr. Davis said that’s more a result of the particular states in play this year, rather than any effort by Democrats.

“The House races are fought in basically red districts, and a lot of Democrats have been hanging out in red districts and voting blue,” he said. “The Senate problem is a lot of the key races — Wisconsin, Connecticut, Washington — these are deep-blue districts.”