WASHINGTON — Democrat Barack Obama has opened a big lead among Hispanic voters, winning support from the vast majority of those who had voted for rival Hillary Rodham Clinton in the Democratic primaries, according to a poll released today.

The national survey, conducted by the nonpartisan Pew Hispanic Center, showed that 66 percent of Hispanic registered voters supported Obama, compared to 23 percent for Republican John McCain. The other 11 percent were undecided.

More than three-quarters of Latinos who had voted for Clinton now say they are for Obama. Clinton carried the Hispanic vote, an important Democratic constituency, by about a 2-1 margin in the primaries.

"Hispanics seem to have seamlessly shifted from Clinton to Obama," said Susan Minushkin, deputy director of the Pew Hispanic Center, a research group based in Washington.

While Hispanics make up only about 9 percent of eligible voters, they could play an important role in four potential battleground states: Florida, Colorado, New Mexico and Nevada.

Both candidates gave speeches before national Hispanic organizations earlier this month. On Wednesday, the Obama campaign released a Spanish language radio ad that will air in the four competitive states.

Hispanics have long supported Democratic candidates, though President Bush started to make inroads, picking up about 40 percent of the Latino vote in 2004.

"We know that Hispanics believe that the Democratic Party is better for Latinos," Minushkin said. "We know that on a variety of issues, Hispanics say that Obama is better" than McCain.

Some Latino advocates argue that Republicans have alienated Hispanics by staking out tough positions against illegal immigration. McCain and Obama both support comprehensive plans to overhaul the immigration system, including a path to citizenship for the estimated 12 million immigrants in the U.S. illegally.

Education, not immigration, was the most important issue cited by Hispanic voters in the Pew survey.

The telephone survey was conducted from June 9 through July 13. The Pew Hispanic Center interviewed a nationally representative sample of 2,015 adult Hispanics, including 892 who said they were registered voters. The margin of sampling error for registered voters was plus or minus 4.4 percentage points.