WASHINGTON (Reuters) - The 2008 Democratic presidential race has tightened, with Barack Obama gaining on front-runner Hillary Clinton six weeks before the first contest, according to a national Reuters/Zogby poll released on Wednesday.

Among Republicans, former New York Mayor Rudy Giuliani expanded his national lead over second-place rival Fred Thompson, the former senator and Hollywood actor, as voters begin to focus on the race for the White House.

“This race is just beginning, let alone all over,” pollster John Zogby said.

Clinton led Obama 38 percent to 27 percent in the new poll, a 10-point fall from her 46 percent to 25 percent lead last month. The drop followed a month of attacks on the New York senator from her rivals and a heavily criticized performance in a late-October debate.

Former Sen. John Edwards of North Carolina remained in third place, climbing four points to 13 percent. All other Democratic contenders scored in low single digits, including New Mexico Gov. Bill Richardson at 4 percent.

About 14 percent of Democratic voters nationwide are still uncertain of their choice as the first contest approaches in Iowa, which kicks off the state-by-state battle to pick candidates for the November 4, 2008, presidential election.

The poll was similar to several other national and state surveys showing Obama, a first-term Illinois senator, gaining on Clinton, the senator from New York who has led most polls all year.

“Clinton had a bad couple of weeks and as a front-runner she’s a target for everyone, she’s treated almost as the incumbent,” Zogby said.

The Reuters/Zogby poll was taken November 14-17, sandwiching the November 15 Democratic debate in Las Vegas, Nevada, where Clinton gave a stronger performance and fired back at her rivals.

In the Republican race, Giuliani widened his lead over Thompson to 14 points, 29 percent to 15 percent, compared to last month’s 28 percent to 20 percent lead.

Former Arkansas Gov. Mike Huckabee jumped over former Massachusetts Gov. Mitt Romney into third place. Huckabee had 11 percent, with Romney and Arizona Sen. John McCain at 9 percent.

A growing number of Republicans, 21 percent, said they have not made up their mind, leaving room for more changes in the field. The shifting numbers, after months of a relatively static race, could indicate voters around the country are beginning to pay attention to the 2008 race, Zogby said.

“There is a real fluidity to both races,” he said.

Democratic presidential candidate Senator Barack Obama (D-IL) speaks to students, voters and supporters at a campaign stop at Prospect Mountain High School in Alton, New Hampshire November 20, 2007. REUTERS/Brian Snyder

Clinton led Obama by at least 20 points among voters age 35 and older. Obama’s strength was with younger voters, leading Clinton by more than 30 points among voters between the ages of 18 and 34.

Obama, who would be the first black president, led by 14 points among black voters. Clinton, who would be the first woman president, led by 18 points among women. They were virtually tied among men.

Giuliani, who has taken heat from social conservatives for his support for abortion rights, led the Republican contenders among voters who described themselves as conservative with 28 percent. Thompson was second among conservatives with 13 percent.

Among those voters who described themselves as “very conservative,” however, Thompson and Huckabee led Giuliani. Thompson drew support from 28 percent of those voters, with Huckabee at 22 percent and Giuliani at 19 percent.

The telephone poll surveyed 545 likely Democratic primary voters and 503 likely Republican primary voters, with a margin of error of plus or minus 4.3 percentage points for Democrats and 4.5 percentage points for Republicans.

(To read more about the U.S. political campaign, visit Reuters "Tales from the Trail: 2008" online at blogs.reuters.com/trail08/)