* Poll: 48 pct say Obama won, 33 pct say Romney
* Impact in opinion polls won't show up until Thursday
* Obama approval rating rises 5 percentage points (Adds detail, byline)
U.S. voters say that President Barack Obama performed better than Republican rival Mitt Romney by a substantial margin in their second debate, according to a Reuters/Ipsos poll released on Wednesday.
Forty-eight percent of registered voters gave the victory to Obama, while 33 percent say Romney prevailed in the Tuesday debate, the online poll found.
The poll reflects the broad consensus of debate observers who said Obama's forceful approach gave him the upper hand over Romney, who was widely seen as the victor in their first matchup on Oct. 3.
"Clearly, the debate was a bit of a turnaround for Obama. He put in a much stronger performance than he did in the first debate and it's showing in the numbers," said Ipsos pollster Julia Clark.
Obama's favorability rating climbed five percentage points after Tuesday's debate to 55 percent. Romney's favorability rating fell two percentage points, to 48 percent.
Voters' views of Obama also improved slightly on a range of issues and personal attributes, from managing the economy to whether he is tough enough for the job.
Obama launched aggressive attacks against Romney on jobs, energy and Libya in the debate. His feisty performance thrilled Democratic supporters who had been disappointed with his lackluster effort in the first debate.
Any impact on the Nov. 6 election will not show up in opinion polls until Thursday at the earliest, Clark said.
Conventional wisdom holds that debates rarely affect the outcome of presidential elections, but this year may prove an exception.
Romney's strong performance in the first debate silenced critics on the right and turned the page on a string of missteps that had hurt him in opinion polls.
Romney surged ahead of Obama in polls in the weeks following the first debate, but his lead was already shrinking before the second debate, which was held at Hofstra University in Hempstead, New York.
As of Tuesday, Obama held a lead of three percentage points in the daily Reuters/Ipsos tracking poll.
The current result is a more accurate reflection of the state of the race than the seesawing polls over the past month, Clark said.
"This was always going to be a very close election and that's back where we are now," she said.
The accuracy of Reuters/Ipsos online polls is measured using a credibility interval. In this case, the survey of 655 online voters, conducted shortly after the debate, has a credibility interval of plus or minus 4.4 percentage points.