WASHINGTON (Reuters) - Democrat Barack Obama has opened a 7-point lead over Republican rival John McCain with five days left in the race for the White House, according to a Reuters/C-SPAN/Zogby poll released on Thursday.

Obama leads McCain by 50 percent to 43 percent among likely voters in the three-day national tracking poll, building on his 5-point advantage on Wednesday. The telephone poll has a margin of error of 2.9 percentage points.

It was the second consecutive day Obama’s lead has grown as the two-year presidential battle draws to a close. McCain is struggling to overtake Obama’s lead in every national opinion poll and in many battleground states.

“This is not good news for McCain. The race was tightening for a few days but now it is going back the other way,” pollster John Zogby said.

Support for Obama, a first-term senator from Illinois, hit or exceeded the 50 percent mark for the seventh time in the last 10 days. McCain’s support has not reached 46 percent in more than three weeks of polling.

Obama held steady or expanded his edge among several crucial blocs of swing voters, leading by 19 points among independents, 10 points among women, 9 points among Catholics and 7 points among voters above the age 65.

Obama also moved ahead of McCain, an Arizona senator, by 5 points among men. McCain still leads by 8 points among white voters but only earns the support of about 30 percent of Hispanics, a fast-growing group that gave President George W. Bush more than 40 percent of their vote in 2004.

The poll also found Obama was doing a better job of reaching across ideological lines, earning the support of nearly 20 percent of self-described conservatives. McCain wins about 10 percent of liberals.

Both candidates campaigned in Florida on Wednesday as Obama simultaneously ran a 30-minute advertisement on three national television networks. He also appeared at a late-night rally in Orlando with former President Bill Clinton -- their first joint campaign event.

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Independent Ralph Nader received 2 percent in the national survey, and Libertarian Bob Barr was at 1 percent. About 2 percent of voters remain undecided.

The rolling tracking poll, taken Monday through Wednesday, surveyed 1,202 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 U.S. 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 Doina Chiacu)