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Polls show that Sen. Mark Begich, Alaska Democrat, is holding on to a 5 percentage point lead over former state Attorney General Dan Sullivan, down from a high of 12 percentage points in July. In Colorado, Sen. Mark Udall a Democrat, is leading Rep. Cory Gardner by an average of 1 percentage point, according to Real Clear Politics. In Louisiana, Sen. Mary L. Landrieu, a Democrat, is ahead of her strongest Republican challengers, Rep. Bill Cassidy and Rob Manness.

Sen. Mark L. Pryor, Arkansas Democrat, is locked in a neck-and-neck race with freshman Rep. Tom Cotton. In Iowa, Rep. Bruce L. Braley, a Democrat, is in a virtual tie with Republican state Sen. Joni Ernst.

Senate Minority Leader Mitch McConnell of Kentucky is clinging to a 3 percentage point lead over his Democratic challenger, Alison Lundergan Grimes, though he took a hit over the weekend when his campaign manager, Jesse Benton, stepped down because of a scandal involving his work with Ron Paul’s presidential campaign.

In Virginia, Republican Ed Gillespie, a former adviser to Mr. Bush, has struggled to gain any traction against the Democratic incumbent, Sen. Mark R. Warner.

On the other side of the ledger, Republicans added some races to the playing field that Democrats considered out of reach earlier this year. Michigan could vote for a Republican for Senate for the first time in 14 years. If the elections turn into a Republican wave, New Hampshire, Minnesota and Virginia also could flip.

“There are a lot of Republican opportunities and very few Democrat opportunities,” Mr. Rothenberg said. “I would describe it as a classic midterm election with an unpopular president where there is strong breeze behind Republican candidates. Will it turn into hurricane-force winds? We will have to wait and see, but it is pretty strong right now.”

Over the summer, a hard-charging Scott Brown, a former Republican senator from Massachusetts, closed a double-digit deficit with Sen. Jeanne Shaheen, New Hampshire Democrat.