Many conservatives attempted to frame Mr. Hoffman’s defeat as a victory, saying that despite Mr. Hoffman’s loss, conservatives prevailed because the moderate Republican candidate, Dede Scozzafava, was forced out of the race.

“Our number one goal was to make clear that the Republican Party cannot take someone as liberal as Dede Scozzafava and thrust her out on the voters and expect the voters just to accept it,” said Brian Brown, executive director of the National Organization for Marriage, which worked to defeat Ms. Scozzafava, the Republican candidate who faced a challenge from Mr. Hoffman.

Democrats had thrown muscle behind the race as well, eager to avoid a potentially embarrassing defeat as President Obama’s approval ratings have softened and efforts to portray them as the party of big government and deficit spending appear to be sticking. A win in the Republican-leaning 23rd Congressional District would provide Democrats with a welcome boost, while a loss would reinforce the notion that the party is struggling.

The seat became vacant after President Obama appointed its long-serving Republican congressman, John M. McHugh, as secretary of the Army.

Leading conservative voices  including The Wall Street Journal’s editorial page and The Weekly Standard and the talk show personalities Rush Limbaugh and Glenn Beck  took on the Republican nominee, Assemblywoman Scozzafava, who supports gay rights and abortion rights and had embraced some Democratic economic policies like the federal stimulus package. They labeled her as too liberal.

The attacks on Ms. Scozzafava eventually took their toll, and she stunned her party over the weekend first by withdrawing from the race and then by urging her supporters to vote for Mr. Owens, a 60-year-old lawyer from Plattsburgh.

Image The White House became involved in the efforts to boost the Democratic candidate, Bill Owens, in the closing days of the campaign. The 23rd Congressional District leans Republican. Credit... Nathanial Brooks for The New York Times

But the ballots had already been printed, and early results showed her picking up 6 percent of the vote. It was unclear how many of those were protest votes, and how many simply did not know she had left the race.