WASHINGTON (Reuters) - Democrat Barack Obama has expanded his national lead over Republican John McCain in the U.S. presidential race to 6 percentage points, according to a Reuters/C-SPAN/Zogby poll released on Monday.

Obama leads McCain 50 percent to 44 percent among likely U.S. voters in the latest three-day tracking poll, up from Obama’s 3-point advantage on Sunday. The telephone poll has a margin of error of 2.9 percentage points.

The rally by Obama broke a string of three consecutive days when McCain had gained ground on the Illinois senator after their final debate on Wednesday. It was the first time in 14 days of the tracking poll that Obama has reached 50 percent.

“Obama has really consolidated his base, and now has huge leads among young people, African-Americans and Hispanics,” said pollster John Zogby.

“Reaching 50 percent puts him in winning territory.”

Obama also increased his support among two key swing groups that could be vital in the November 4 election. His edge with independents rose from 8 points to 11 points, and his lead among women grew from 6 points to 8 points.

McCain narrowly trails Obama among men and leads by 13 points, 53 percent to 40 percent, among whites.

“McCain seems to have slipped a little bit, but in the grand scheme it’s still a very close race,” Zogby said.

Obama has led McCain, an Arizona senator, by between two and six points in all 14 days of polling. “This race has not really moved all that much in two weeks,” Zogby said.

POWELL ENDORSEMENT

The expanding lead for Obama came as he received the endorsement of Republican former Secretary of State Colin Powell and announced he had raised a stunning $150 million in September.

His fundraising haul shattered the records he already owns and will fuel a huge advantage for Obama in paid advertising in the final 15 days of the campaign.

Some other tracking polls also showed the race tightening in the last few days, but with the help of his huge spending advantage Obama has continued to hold an edge on McCain in some key battleground states.

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The poll, taken Friday through Sunday, showed independent Ralph Nader and Green Party candidate Cynthia McKinney with 1 percent support. Libertarian Bob Barr barely registered any support.

The rolling tracking poll surveyed 1,211 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 in an effort to track changing momentum.

The U.S. president is determined not by who wins the most national votes but by who wins the Electoral College, which has 538 members apportioned by population in each state. Electoral votes are allotted on a winner-take-all basis in all but two states, which divide them by congressional district.