“Democrats believe we must permanently extend tax cuts for the middle class before they expire at the end of the year, and we will,” said Jim Manley, a spokesman for the majority leader, Harry Reid of Nevada. “Unfortunately, to this point we have received no cooperation from Republicans to do so.” Mr. Manley added, “We will come back in November and stay in session as long as it takes to get this done.”

But Republicans said the accusations of obstruction were ridiculous given that Democrats had not even tried to bring legislation to the floor to extend the tax cuts.

“It’s odd to complain about a lack of cooperation on a bill that doesn’t exist,” said Don Stewart, a spokesman for the Senate Republican leader, Mitch McConnell of Kentucky.