U.S. Republican Mitt Romney staged a strong debate performance Wednesday, outshining his rival President Barack Obama. But the debate could barely be a game-changer, an expert says.

Peter Hanson, an assistant professor of politics at Denver University, told Xinhua that Romney came into the debate "needing to prove himself, he needed to show his passionate side, trying to make up some damage that had been done by his video tape about 47 percent of Americans."

"He needed to be aggressive, showed he can take on President Obama, and land a few punches. I think he did both of those things tonight," said Hanson, whose university hosted the debate.

With under five weeks to go until the Nov. 6 election, Romney was still trailing in polls when heading into the debate, eager to turn the tides in his campaign trail marred by several blunders, including his videotaped remarks scorning almost half of the nation's voters.

A hidden-camera video tape showed Romney disparaging 47 percent of the Americans, saying they don't pay taxes while considering themselves as "victims."

Two-thirds of people who watched the presidential debate reckoned Romney won the showdown, while only one fourth considered Obama as the winner, according to a CNN/ORC International poll.

"The president seemed flat," said Hanson. "He passed up some opportunities to clearly define the differences between himself and Mitt Romney. He passed up some opportunities to land some punches on Romney."

But that doesn't mean Romney will somehow turn the table. There are still two more debates to go, and little evidence shows debates could too much sway with voters.

The president is ahead in the polls and debates can move opinion only by historically one or two points, Hanson said.

"Even though Romney did well tonight, it remains to be seen how much of a gain in the poll he gets from this. It may still not be enough to overcome Obama's lead in some of the key swing states," he said.

And the campaign has a major role to play in shaping the discussion after the debate.

"The effect on the public is really going to be seen in the next few days," with a lot of the undecided who didn't watch the debate forming their opinion with what they read on paper and hear from friends.

Moreover, the case where Romney easily came out as better prepared is not likely to happen again.

This debate "is certainly been a wake-up call for the president," said Hanson. "He's gonna have to be a little more lively and a little more aggressive on the next debate."