How big an impact did Mitt Romney's performance in last week's debate have? Huge. Mr. Romney not only won the night, he changed the arc of the election—and perhaps its outcome. Surveys have him leading the RealClearPolitics average of polls for the first time since securing the GOP nomination in mid-April.Prior to Oct. 3, Mr. Romney trailed President Barack Obama by an average of 3.1 points in national polls tallied by RealClearPolitics. Since the debate, Mr. Romney now leads Mr. Obama in the RCP average by a point, 48.2% to 47.2%, and the bounce is likely to grow. By comparison, Sen. John Kerry was widely seen to have bested President George W. Bush in the first 2004 debate (held on Sept. 30 of that year), but he never led in the RCP average in October.In seven of the past nine presidential debate series, the challenger has gained more in the polls than the incumbent (or the candidate of the party in power). The first debate generally frames the series and establishes whether the bounce will be large or modest. Mr. Romney's bounce is significant.It's unlikely that Mr. Obama will do as poorly next Tuesday at Hofstra University in New York. His supporters are demanding that he be more aggressive. He will be, telling AM radio's Tom Joyner on Wednesday that he'd been "too polite" in the first debate.But if the president is as angry and negative in the Oct. 16 debate as he has been on the campaign trail the past week, he will damage himself again. It's hard in a town-hall format like next week's to attack, and too easy to come across as mean and nasty. Also, alleging that Mr. Romney is a serial deceiver—as the president and top advisers are doing—is a hard sell. Mr. Romney came across last week as practical and thoughtful, authentic and a straight shooter.A record 72% in the Oct. 8 Gallup survey said he won the debate, compared with 20% who thought Mr. Obama did. Voters would not have awarded such a lopsided victory to a liar.An Oct. 7 Pew Research report found that before the debate, Romney voters were four points more likely than Obama voters to give the election "a lot of thought." After it, Romney-voter engagement was 15 points higher than that of Obama voters. This enthusiasm gap already expresses itself in voter registration and is now influencing early voting.In the eight battleground states that register voters by party, Republicans have maintained their advantage or cut into the Democrats' in all but one (Nevada). Since September 2008, Republicans have kept their registration advantages in Colorado and New Hampshire. They've added more new Republican registrations than Democrats did in Florida, Iowa and North Carolina. And they've lost fewer voters from the rolls than Democrats did in New Mexico and Pennsylvania.Republicans are also getting the better of Mr. Obama in early voting. In 2008, Democrats made up 51% of the North Carolina early vote while Republicans were 30%. This year, Republicans have cast 54% of the ballots returned so far, Democrats only 28%, according to state data compiled by George Mason University's Michael McDonald for his United States Election Project.In Florida, 46% of absentee ballots returned by September's end came from Republicans (compared with 37% in 2008) while just 38% came from Democrats (they were 46% of the total in 2008). More Republicans have requested absentee ballots in Colorado, a state where Democrats edged out Republicans in early voting last time.Republicans have also made up ground in Ohio. For example, in 2008 Democrats requested 5% more absentee ballots in Franklin County (Columbus), 4% more in Greene County (Xenia), and 11% more in Wood County (Bowling Green). This election, Republicans have more ballot requests than Democrats in these counties by 5%, 19% and 1% respectively.The Romney campaign saw a $12 million surge in online contributions following the debate, and major GOP fundraisers are again opening their checkbooks. True enough, Hollywood stars and rich San Francisco liberals wrote big checks during Mr. Obama's two-day California swing this week. But it isn't clear what overall impact the president's poor debate performance will have on his fundraising. The small Internet donors that produced an eye-popping $181 million fundraising total in September may be disappointed in his debate skills and waiting to see if he improves.During the GOP primary, one of Mr. Romney's chief selling points was his skill as a debater. He picked a powerful moment to display this strength. The debate at the University of Denver qualifies as among the most consequential in history. It might end up as the election's decision point. Mr. Rove, a former deputy chief of staff to President George W. Bush, helped organize the political action committee American Crossroads. He is the author of "Courage and Consequence" (Threshold Editions, 2010).