SIMI VALLEY, California (Reuters) - Ten Republican White House contenders, led by early favorites Rudolph Giuliani and John McCain, will crowd the stage on Thursday for their first debate in an already heated 2008 presidential campaign.

A week after the Democratic candidates held their opening debate but more than 18 months before the November 2008 election, the Republicans take their turn in the political spotlight at the presidential library of conservative hero Ronald Reagan outside Los Angeles.

Giuliani, the former mayor of New York who has led Republicans in national polls for months, and McCain, an Arizona senator who was an early favorite but has struggled of late, are well-known political commodities.

But the rest of a largely unknown Republican field will be looking to introduce themselves to primary voters and make a good first impression.

For candidates like former Massachusetts Gov. Mitt Romney, whose strong fund-raising and establishment support have put him in the race’s top tier even as he lingers in single digits in the polls, the debate offers an early opportunity.

“For a lot of people watching, this is their first chance to go comparison shopping with 10 different candidates,” said Kevin Madden, a spokesman for Romney. “Our goal right now is to introduce ourselves to the American public and introduce the governor’s ideas.”