WASHINGTON (Reuters) - Democrat Barack Obama’s lead over Republican rival John McCain has grown to 12 points in the U.S. presidential race, with crucial independent and women voters increasingly moving to his side, according to a Reuters/C-SPAN/Zogby poll released on Thursday.

With less than two weeks before the November 4 election, Obama leads McCain 52 percent to 40 percent among likely voters in the latest three-day tracking poll, which had a margin of error of 2.9 points.

Obama has made steady gains over the last four days and has tripled his lead on McCain in the past week of polling.

“Obama’s expansion is really across the board,” pollster John Zogby said. “It seems to be among almost every demographic group.”

The Illinois senator saw his lead among women -- who are expected to play a decisive role in this election -- increase to 18 points from 16 points on Wednesday.

And independent voters, who have been the target of intense campaign efforts by both sides, have now swung behind Obama by a 30-point margin, 59 percent to 29 percent.

Zogby said McCain, 72, appeared to have lost the traction he won after the third and final presidential debate last week.

“McCain can still try to turn it around, but he has to find focus,” Zogby said, adding that economic issues, which dominated the campaign amid turmoil in the credit, housing and financial markets, still seem to be working in Obama’s favor.

“At some point there are some issues that just overwhelm, and McCain has been particularly weak on the economy,” Zogby said in a statement.

Other recent national polls have given Obama a narrower lead, but Zogby said he was confident in his sampling methods.

The latest poll showed a continued erosion of McCain’s support even among his “base” voters.

While Obama wins the backing of 86 percent of Democrats, only 81 percent of Republicans back the Arizona senator -- down from figures in the low 90s immediately after the Republican national convention in early September.

Obama holds a 6-point lead among men, 48 percent to 42 percent, while white voters -- who had been among McCain’s core support groups -- now only back McCain by a 2-point margin.

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Independent Ralph Nader and Libertarian Bob Barr held relatively steady at 2 percent and 1 percent respectively. Three percent of voters said they remained undecided, unchanged from Wednesday.

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 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.