Senator Hillary Clinton is expected to be the focus of attention Thursday when the Democratic presidential contenders take part in a debate in Las Vegas, Nevada.  VOA National Correspondent Jim Malone has more from Washington.

Clinton heads into the debate leading in public opinion polls, but facing intensified criticism from her Democratic rivals.

At a recent Democratic dinner in the early contest state of Iowa, former Senator John Edwards raised questions about Senator Clinton's credibility. 

"You are in a place to judge who is trustworthy, who is honest, who is sincere," he said.

Clinton has been on the defensive since the last Democratic debate two weeks ago.  Clinton was asked about a plan to grant driver's licenses to illegal immigrants in New York.  At first she seemed to support the idea, then backed away from it.  Clinton's campaign now says she opposes the proposal, which was eventually dropped by New York's governor.

In addition to questioning her credibility, Clinton's rivals have also seized on public opinion surveys that suggest she is the most polarizing contender in the presidential race, liked and disliked by voters in roughly equal numbers.

Senator Barack Obama, who is running second to Clinton in national polls, openly questioned whether she is the strongest Democratic candidate for president on NBC's Meet the Press.

"I believe that it is going to be harder for her to win because I think a lot of voters go in with some preconceptions about her that are going to be very difficult to overcome," he said.

But the intensified attacks seem to have done little damage to Clinton in the polls.  She leads nationally among Democrats and in the early primary state of New Hampshire.  Clinton, Obama and Edwards are in a close race in Iowa, which kicks off the presidential selection process on January 3.

During a recent speech in Iowa, Clinton urged Democrats to focus their rhetorical fire on Republicans, and not each other.

"We should be turning up the heat on the Republicans!  They deserve all the heat we can give them!" she said.

In the Republican race, former New York City Mayor Rudy Giuliani holds his lead in the latest national polls.  But former Massachusetts Governor Mitt Romney continues to lead in the early contest states of Iowa and New Hampshire over Giuliani, Senator John McCain, former Senator Fred Thompson and former Arkansas Governor Mike Huckabee.