LAS VEGAS – Sen. Barack Obama accused Sen. Hillary Rodham Clinton of being duplicitous in her presidential campaign in one of several sharp exchanges that marked the Democratic debate Thursday night as one of the most heated to date.

With less than 50 days until the first primary voting begins, the Democratic contenders showed off newly polished answers to familiar questions. Clinton, D-N.Y., under fire for sounding evasive during the last debate more than two weeks ago, made a noticeable effort to sound definitive and concise.

But at a critical moment in the race – when voters in the early-voting states are beginning to pay full attention – Clinton, Obama and former senator John Edwards spent the opening moments of the debate on the attack.

Obama, D-Ill., described Clinton as “a capable politician,” then added: “But what the American people are looking for right now is straight answers to tough questions. And that is not what we’ve seen out of Senator Clinton on a host of issues.”

Clinton fired back swiftly, citing Obama’s health-care proposal as a sign of his weaknesses as a leader. “Well, I hear what Senator Obama is saying, and he talks a lot about stepping up and taking responsibility and taking strong positions,” Clinton said. “But when it came time to step up and decide whether or not he would support universal health-care coverage, he chose not to do that.”

Clinton accused Obama of omitting about 15 million Americans from his health-care proposal. He interrupted her a few moments later, triggering an intense back-and-forth.

Edwards, of North Carolina, who in past debates was the most aggressive candidate onstage, continued his criticism of Clinton. “The issue is whether we can have a president that can restore trust for the American people in the president of the United States,” Edwards said, drawing applause. “Because I think this president has destroyed that trust, and I think there are fair questions to be asked of all of us, including Senator Clinton.”

Thursday’s debate, the fifth of six forums sanctioned by the Democratic National Committee, was held on the campus of the University of Nevada at Las Vegas and was aired nationally on CNN. Anchor Wolf Blitzer was moderator.

Seven Democrats participated: Clinton, Obama, Edwards, New Mexico Gov. Bill Richardson, Sen. Joe Biden, D-Del., Sen. Christopher Dodd, D-Conn., and Rep. Dennis Kucinich, D-Ohio.

Clinton arrived here under pressure to rebound from what even she conceded was a weak performance in the Democrats’ last debate, Oct. 30 in Philadelphia. Asked that night whether she supported a proposal by New York Gov. Eliot Spitzer to allow illegal immigrants to obtain driver’s licenses, she appeared to endorse the idea and then pulled back. Dodd, Edwards and Obama all sharply rebuked her.

Clinton’s campaign later accused the others of piling on and suggested that she was attacked in part because she is a woman.

But Edwards and Obama kept up the criticism on the license issue as well as others, and Clinton was clearly thrown on the defensive for the first time in the campaign.

The controversy over illegal immigrants and driver’s licenses continued this week. When Spitzer pulled down his proposal on Wednesday, Clinton issued a statement applauding him and noting that, as president, she would oppose such an idea.

That brought more criticism from her rivals. Obama spokesman Bill Burton said Clinton had taken “two weeks and six different positions to answer one question.” Dodd spokeswoman Colleen Flanagan accused her of “flip-flopping cubed,” and Edwards’ spokesman, Eric Schultz, exclaimed, “We’re dizzy.”

Edwards has consistently challenged Clinton in debates, and Obama has shown signs of hitting his stride as a candidate. He delivered an impressive speech at the Iowa Democratic Party’s Jefferson-Jackson dinner on Saturday, implicitly challenging Clinton as too cautious and poll-driven.

“If we are really serious about winning this election, Democrats, then we can’t live in fear of losing,” he said.

Some polls taken after the Philadelphia debate showed Clinton’s lead shrinking nationally as well as in some early-voting states. With Obama and Edwards increasingly aggressive, Clinton’s advisers prepped for another tough session in Las Vegas.

Clinton unveiled a new theme – “turn up the heat” – at the dinner in Iowa, promising to take the fight to Republicans if she is the Democratic nominee. But it was an apt description of what is happening in the Democratic race seven weeks out from the Iowa caucuses.

The debate highlighted the growing role of Western states in Democrats’ calculus. Long a Republican stronghold, the Rocky Mountain West could be a far more competitive region in 2008 than in past elections. Democrats have made gains at the state level in the region, particularly in Colorado, and control the governorships in that state, New Mexico, Arizona, Wyoming and Montana.

Democrats picked Denver for their national convention next year to signal the West’s rising power, and they see Nevada, Colorado and Arizona as potentially competitive states in 2008 in part because of their growing Hispanic populations. Republicans narrowly won New Mexico in 2004, but Democrats see that as their likeliest pickup in the region.

Nevada has emerged as a potentially critical battleground in 2008 because of its place early in the nominating calendar. In an effort to add geographic, demographic and economic diversity to the nomination battle, the DNC originally voted to make the state’s caucuses the second contest of 2008 – after Iowa but before New Hampshire. Events have made it likely that the Jan. 19 caucuses here will be the third contest, after the Iowa caucuses and the New Hampshire primary.

Clinton holds a strong lead in early polls in the state, but because the turnout could be so low, those polls are somewhat suspect. The shape of the race likely will be affected by what happens in Iowa and New Hampshire. One key to winning the caucuses here will be grabbing the endorsement of the Culinary Workers Union, the biggest union in the state, with roughly 60,000 members. Edwards had hoped for their support earlier this year, but the battle now appears to be between Clinton and Obama.

AP-NY-11-15-07 2224EST

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