WASHINGTON (Reuters) - Democrat Barack Obama has opened a 6-point lead over Republican rival John McCain in the U.S. presidential race, according to a Reuters/C-SPAN/Zogby poll released on Sunday.

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Obama leads McCain by 49 percent to 43 percent among likely U.S. voters in the latest four-day tracking poll, his widest lead since the poll was started on Tuesday. It was up from a 4-point lead on Saturday. The poll has a margin of error of 2.9 percentage points.

Pollster John Zogby said Obama’s lead was now statistically significant.

“As we watch each day, it is clear that he has gone from a 2-point lead to a 3-point lead on up to 6 points,” Zogby said. “It is certainly trending his way.”

With just over three weeks to go before the November 4 election, the poll showed Obama gaining traction among independent voters who now back him by a 21-point margin.

Among women, another crucial group, the Illinois senator held a solid 12-point lead, while the two candidates were tied among male voters at 45 percent apiece.

Obama has widened his lead as weeks of economic turmoil shook financial markets, causing stock markets to plunge and fueling voter concern over pocketbook issues.

Young voters aged 18 to 29 backed Obama by a 20-point margin, and he also held a double-digit lead among those who reported they had registered to vote in the past six months.

McCain had a 10-point lead among white voters, while Obama, who would be the first black U.S. president, won the support of 92 percent of black voters, one of the Democratic Party’s most loyal constituencies.

Zogby said McCain, a former Navy pilot and Vietnam prisoner of war, appeared to be failing to connect with many voters on the issue of the economy, and a wave of attacks leveled against Obama by the McCain campaign also fell flat.

“Clearly the negative campaigning isn’t working,” Zogby said, noting that Obama was winning support among some voters in even the strongest traditional Republican voting blocs.

“He’s getting 19 percent support among conservatives and 35 support among born-again or evangelical (Christian) voters, which is pretty substantial. I wonder if the McCain campaign ought to be raising some red flags,” he said.

The rolling tracking poll surveyed 1,206 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.