DENVER, Colorado — Republican challenger Mitt Romney came out firing Wednesday in his first presidential debate, attacking Barack Obama for economic policies he said had "crushed" the American middle class.

"I'm concerned that the path we're on has just been unsuccessful," said Romney, fighting for his political life as he seeks to turn around a flagging campaign that has him trailing in key states just weeks before election day.

"The president has a view very similar to the view he had when he ran four years ago, that a bigger government, spending more, taxing more, regulating more -- if you will trickle-down government -- would work," Romney said.

"That's not the right answer for America. I'll restore the vitality that gets America working again," he vowed. "Middle-income families are being crushed, and the question is, how to get them going again."

Obama hit back by suggesting that Romney will bring in $5.4 trillion in tax cuts, particularly geared towards the wealthy, and said Romney hadn't been clear about what loopholes in the tax system he would close.

"Governor Romney has a perspective that says if we cut taxes skewed toward the wealthy and cut back regulations, we'll be better off. I have a different view," Obama said, calling for "economic patriotism."

Romney, the former governor of Massachusetts, fought back hard against Obama's claims as the debate saw its first real clash of the night.

"Virtually everything he said about my tax plan is inaccurate. If the tax plan he described were a tax plan I was asked to support, I would say absolutely not," he said, adding: "I'm not looking for a $5 trillion tax cut."

This led the Democratic incumbent to accuse his Republican challenger of backing away from his campaign pledges, as what started as a cordial series of exchanges descended into a fierce political exchange.

The president, who appeared the more nervous of the two in the opening exchanges, also sought to remind Americans that he inherited a terrible situation from former Republican president George W. Bush.

"When I walked into the Oval Office, I had more than a trillion dollar deficit greeting me. And we know where it came from," Obama said.

"Two wars that were paid for on a credit card. Two tax cuts that were not paid for. And a whole bunch of programs not paid for and then a massive economic crisis."

After hundreds of campaign stops, $500 million in mostly negative ads and countless tit-for-tat attacks, Obama and Romney were going head to head in their debut debate.

Both candidates had completed walkthroughs of the venue hours before the first of three televised showdowns just 33 days before American voters decide their fates.

Obama clings to a narrow lead in his bid to defy the omens sown by a stubbornly sluggish economic recovery and to become only the second Democrat since World War II to win a second term.

Romney, down in almost all the key battleground states that will decide who wins the 270 electoral votes needed to win on November 6, seeks a sharp change of momentum in a race that seems to be slipping away from him.

The debate was set to focus on economic issues but veteran anchor Lehrer, who was steering the debate for tens of millions of viewers at home, had leeway to bring up other subjects.

That means Obama, 51, could face a grilling on his administration's shifting account of the attack on the US consulate in Benghazi, Libya on September 11.

Romney, a multi-millionaire former venture capitalist, could come under scrutiny over his complex offshore tax arrangements, which Democrats have highlighted to press the case that he is indifferent to middle-class struggles.

The 65-year-old badly needs to reset the election narrative, after a video emerged of him branding 47 percent of Americans as people who pay no taxes and see themselves as "victims" who depend on government handouts.

The president was marking his 20th wedding anniversary on Wednesday and began the debate with a shout-out to the First Lady, apologizing for the unromantic setting.

"Congratulations to you, Mr. President, on your anniversary. I'm sure this is the most romantic place you could imagine, here with me," joked his adversary in a rare moment of levity as the duel began.

Republicans sought to capitalize before the debate on a verbal slip on Tuesday by Vice President Joe Biden, who said the middle class had been "buried" for the last four years.

Democrats said Biden was talking about how president Bush's policies continued to hurt the middle class deep into Obama's term.

Romney's campaign released a Web video showing Biden's comments, followed by the simple line: "We couldn't have said it better ourselves."

Romney re to the theme, saying in the debate: "Under the president's policies, middle-income Americans have been buried, have been crushed."

Obama countered with accusations that Romney has no specific plans to create jobs or move the country forward, other than the failed Republican policies that he said got the country in trouble in the first place.

Several national polls released before the debate showed a tight race, with Obama ahead by a few points.

A new Wall Street Journal/NBC News poll gave Obama a lead among likely voters of 49-46 percent, consistent with a RealClearPolitics poll average showing the graying US leader up by 3.5 percentage points.

A National Public Radio poll showed Obama leading 51-44 among likely voters nationwide and 50-44 in battleground states, while a Washington Post-ABC News poll gave Obama a larger 52-41 lead in swing states.