A Siena Research Institute Poll on Tuesday showed the Republican nominee trailing Mr. Obama by only three percentage points, 43 percent to 40 percent, in the Keystone State. Other polls prior to the debate had shown Mr. Obama with as much as a 12-point lead.

“Pennsylvania’s 20 electoral votes suddenly appear to be up for grabs,” said poll director Don Levy. “Romney not only has Republican support but now leads in vote-rich areas outside of Philadelphia and also in the central part of the state. With a month and two more debates to go, Pennsylvania’s direction on the road to the White House remains in doubt.”

The Siena survey found 12 percent of the state’s voters undecided. The poll was conducted from Oct. 1 to Oct. 5; the debate took place on Oct. 3.