ABINGDON, Va.—Mitt Romney's strong debate performance Wednesday night has generated $12 million in online contributions, his campaign said, as well as a surge in volunteers and bigger crowds at his events.

The challenge for Mr. Romney in the weeks ahead will be translating this jolt of energy into votes in a race that saw the Republican nominee enter the first debate trailing President Barack Obama in most of the states that will decide the election.

Republicans and Democrats say Mr. Romney's performance before a television audience of some 67 million people bought the GOP nominee a second look from swing voters and may help him improve an image battered by negative ads and his own missteps. The debate also gave Mr. Romney a chance to do something he couldn't in the primary: trumpet his record of working with Democrats and present himself as a practical deal maker who favors results over ideology.

One former Obama White House official said the debate "has caused people to take a second look at Romney."

It is too early to tell how much Mr. Romney moved the needle Wednesday night, and the early evidence is more anecdotal than empirical: Volunteers in Nevada lined up before campaign staff could open the doors on Thursday morning, the campaign said, and larger-than-usual groups showed up to work at field offices across the country. The campaign said it added more than 300,000 new Facebook friends.