Straggling members of Congress bolted from Washington on Thursday for the campaign trail, and if Senate leaders can be believed, it is pretty rough out there.

“It is sort of like a knife fight in a phone booth,” said Senator Mitch McConnell, the Kentucky Republican and majority leader, about the razor-close nature of the handful of races that will decide control of the Senate.

He and Senator Chuck Schumer, the New York Democrat who stands to assume the majority leader post if the party is able to mount a takeover, acknowledged that the Senate is up for grabs, with Democrats on offense.

Mr. Schumer predicted a Democratic victory, but he said that the possibility of a win for his side had been made more difficult by an influx of money behind Senate Republican candidates from the political network headed by the conservatives Charles G. and David H. Koch, who are focusing more on the Capitol than on the White House.