Distressed Democrats Look to Obama to Boost Turnout

Perhaps most exasperating to Democrats is the Senate race in Georgia, where a second woman this week accused Mr. Walker, the former football star, of having paid for her to have an abortion. Mr. Walker has denied both women’s claims.

Caught on a hot microphone on Thursday, Senator Chuck Schumer of New York, the majority leader, was heard telling Mr. Biden, “The state where we’re going downhill is Georgia. It’s hard to believe that they will go for Herschel Walker.”

Jessica Taylor, the Senate analyst at the nonpartisan Cook Political Report, said, “It really does feel like there is a lot of movement in these races.” She still rates control of the Senate a tossup, but, she added, “If this is a wave, you could see others swept away,” like Senator Michael Bennet, Democrat of Colorado.

On the other hand, recent polling has tantalized Democrats in Iowa, where Senator Charles E. Grassley, a Republican, is seeking another six-year term at age 89; in North Carolina, where the Democratic nominee for the Senate, Cheri Beasley, remains knotted with her Republican opponent, Ted Budd, and even in Utah, where Senator Mike Lee, a Republican, is facing an unexpectedly strong challenge from an independent, the former C.I.A. officer Evan McMullin.

“This is the trickiest midterm environment I’ve seen in a long time,” said Steve Israel, a former House member from New York who once headed the House Democrats’ campaign arm.

Maya King reported from Atlanta and Jonathan Weisman from Chicago.