WASHINGTON (Reuters) - Democrat Barack Obama has opened an 8-point lead over Republican John McCain two weeks before the U.S. presidential election, according to a Reuters/C-SPAN/Zogby poll released on Tuesday.

Obama leads McCain 50 percent to 42 percent among likely U.S. voters in the latest three-day tracking poll, up from a 6-point advantage for Obama on Monday. The telephone poll has a margin of error of 2.9 percentage points.

“It was another very big day for Obama,” said pollster John Zogby. “Things clearly are moving in Obama’s direction.”

It was the second consecutive day that Obama gained ground on McCain as the two head into the final sprint to the November 4 election.

Obama, an Illinois senator, expanded his lead among two key swing groups. His advantage with independent voters grew from 11 to 15 points, and his edge with women voters grew from 8 to 13 points.

Obama also took a lead among voters above the age of 70 and expanded his lead among Hispanics and Catholics. His support among Republicans grew from 9 percent to 12 percent a day after he received the endorsement of Republican former Secretary of State Colin Powell.

“Maybe this is the Powell effect,” Zogby said. “That wasn’t just an endorsement, that was a pretty powerful statement.”

McCain narrowly trails Obama among men and saw his lead among whites drop from 13 points to 9 points, 51 percent to 42 percent. Zogby said Obama was doing better than 2004 Democratic presidential candidate John Kerry among crucial sub-groups.

“He is clearly outperforming Kerry,” Zogby said. “But two weeks is a lifetime in politics.”

This was the first time Obama has stretched his advantage over McCain, an Arizona senator, to more than 6 points since the tracking poll began more than two weeks ago. Obama’s edge had been between 2 and 6 points in all 15 days of polling.

Some other tracking polls have showed the race tightening in the last few days. But with the help of his huge spending advantage, Obama has maintained an edge on McCain in key states.

The poll, taken Saturday through Monday, showed independent Ralph Nader gaining 2 percent support. Green Party candidate Cynthia McKinney and Libertarian Bob Barr each registered 1 percent support.

The rolling tracking poll surveyed 1,214 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.

Slideshow ( 22 images )

The U.S. president is determined 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.