WASHINGTON — Sal Pace, the Democratic leader of the State House in Colorado, was already preparing to run for Congress in a district captured by Republicans last year, but his party’s special election victory last week in a conservative district in upstate New York made the decision all the easier.

“The New York race confirmed what I thought citizens would feel about Medicare,” said Mr. Pace, who is expecting to soon begin a campaign to oust Representative Scott Tipton, a freshman Republican, in southwestern Colorado. “People are very hesitant to end Medicare as we know it.”

In the aftermath of the New York victory, which hinged on a Republican plan to reshape the health care program for older Americans, members of both parties and independent analysts now predict a more competitive race next year for control of the House, with expanded opportunities for Democrats to reclaim seats they lost in the Republican wave of 2010.

“No one believes that 2012 will be the same type of election for Republicans with the wind at our back,” said Tom Reynolds, a former New York congressman who ran the Republican Party’s House campaign effort in 2004 and 2006. “Neither will it be ’08 or ’06 with the wind in our face. Republican office holders will have to go out and explain what the problem is and what they are trying to do to fix it.”