US Republicans were quick to claim bragging rights after Mitt Romney's aggressive performance in the first presidential debate.

Meanwhile, Democrats struggled to explain President Barack Obama's often flat showing.

Within minutes of the debate's end, Republicans rushed into the media room to praise Mr Romney and declare that his struggling presidential campaign had turned a corner.

"It's a whole new ballgame," Republican Senator John Thune said of the White House race after Mr Romney's performance.

"This was a chance for him to discuss his plans in a straightforward way with his own message, unfiltered," Mr Thune said. "I thought he crushed it."

Democrats took their time before greeting the media, with a handful of advisers and campaign staff scrambling to explain the president's muted and strangely reticent showing.

Mr Obama delved into policy in great and rambling detail, but never reminded voters of two of his most effective attacks against his rival: Mr Romney's experience at the Bain Capital private equity fund and the secretly recorded "47 percent" video showing him criticizing nearly half the electorate.

"The president wasn't looking at a checklist of attack lines. He was trying to explain his plans," said campaign spokeswoman Jen Psaki.

Campaign manager Jim Messina said Mr Obama was not worried about trying to shoehorn the attacks on Mr Romney into a fact-filled and detail-heavy debate on economic policy.

"We've always treated the American voters with respect," White House adviser David Plouffe said when asked if Mr Obama had been too detailed. "Are you saying they can't understand details of his healthcare plan, or of Medicare?"

Mr Messina, asked if the famously wordy Mr Obama would work on keeping his answers shorter, smiled and said, "That's never going to be our strong suit."

Mr Obama's weak first performance was in keeping with other recent first debates for incumbent presidents. George W Bush in 2004, his father, George HW Bush, in 1992 and Ronald Reagan in 1984 all had difficult first debates after being deferred to during their first four years as president.

Republican Rudy Giuliani, former mayor of New York, said Mr Obama's unease showed.

"This is the first time in four years, since he debated Hillary Clinton, that anyone has really challenged this man, and he fell apart," Mr Giuliani said, describing Mr Obama at times as "totally befuddled."

Mr Romney had taken a lot of time off the campaign trail to prepare for the debate, and his strong performance justified that decision, Republicans said.

He entered the debate needing a good showing to steady his campaign. He had fallen behind Mr Obama in polls, and nervous Republicans had begun to worry his weak performance at the top of the ticket would hurt them in their battle for Congress as well.

But Mr Thune said Republicans across the country would take heart in the debate. "This one was really important because it was the first one," he said.