By John Whitesides

WASHINGTON (Reuters) – Democrat Barack Obama has expanded his national lead over Republican John McCain in the presidential race to 10 points, according to a Reuters/C-SPAN/Zogby poll released on Wednesday.

Obama leads McCain 52 percent to 42 percent among likely U.S. voters in the latest three-day tracking poll, up from an 8-point advantage for Obama on Tuesday. The telephone poll has a margin of error of 2.9 percentage points.

It was the third consecutive day Obama gained ground on McCain as the two begin the final sprint to the November 4 election.

“Obama just keeps growing, he has expanded his lead among almost every major voting group,” said pollster John Zogby. “McCain seems to be out of steam for the moment.”

The 10-point lead was the first time Obama’s advantage over McCain, an Arizona senator, reached double-digits in the poll. Obama’s lead had floated between 2 and 6 points in the more than two weeks of polling until stretching to 8 points on Tuesday.

Obama made gains with two key swing voting blocs. His advantage with independent voters grew to a whopping 27 points from 15 points and his edge with women voters grew to 16 points from 13.

Obama, an Illinois senator, led among all age groups and in every income group except for the most wealthy voters. He now has the support of 21 percent of self-described conservatives — his best showing with those voters.

McCain narrowly trails Obama by 2 percentage points among men and saw his lead among whites drop to 6 points from 9 points, 50 percent to 44 percent.

The poll, taken Sunday through Tuesday, showed independent Ralph Nader, Green Party candidate Cynthia McKinney and Libertarian Bob Barr each registering 1 percent support.

Three percent of voters remain undecided.

The rolling tracking poll surveyed 1,208 likely voters in the presidential election. In a tracking poll, the most recent day’s results are added while the oldest day’s results are dropped to monitor changing momentum.

The president is determined by who wins the Electoral College, which has 538 members apportioned by population in each state and the District of Columbia. Electoral votes are allotted on a winner-take-all basis in all but two states, which divide them by congressional district.

(Editing by Peter Cooney)