Republicans have lost all of the gains among Hispanic voters that four years ago helped vault President Bush into the White House, a new poll shows, a reversal that could jeopardize the party’s chances to win the presidency in 2008. The study, by the nonpartisan Pew Hispanic Center, shows Hispanic voters cooling off to Republicans and veering toward the Democrats, ending a trend that since 1999 had Republicans winning over increasing numbers of Hispanic voters.

Part of the reason may be the increasing importance of the immigration issue to this small but powerful voting bloc, which is strategically situated in several key swing states, according to the study’s authors, Paul Taylor and Richard Fry of the Pew Hispanic Center.

The study suggests that an escalating competition among Republican presidential hopefuls to stake out the toughest stance against illegal immigrants could end up spoiling the party’s chances of winning the White House next year.

“There is a long way to go until the 2008 election, but the Pew survey of Latinos finds a number of potentially worrisome early signs for the GOP,” the study said. Immigration is now a central issue to nearly 80% of Hispanic voters, compared to 63% in 2004. Forty-one percent of Hispanic voters say Democrats are doing a better job dealing with illegal immigration, compared to 14% who support the Republicans’ handling of the issue, according to the study.

Hispanic voters also tend to believe that Democrats care about them more than Republicans do, the study found.

In 1999, the percentage of Hispanics leaning toward the Democrats had a 33-point lead over the percentage aligned with Republicans. That gap closed to 21 percentage points by last summer, just as a bipartisan bill to overhaul the national immigration system was falling apart after some Republicans publicly rallied against it. They argued that it would constitute amnesty for millions of illegal immigrants.

Since July, the gap between Hispanics who lean Democrat and those who lean Republican has increased to 34 percentage points. Daniel Schnur, a GOP observer and former communications director for Senator McCain, called the drop in Hispanic support for Republicans a “natural reaction.” “There’s no question that there’s a potential for Hispanic backlash as a result of the immigration debate,” he said.

He noted, however, that non-Hispanic swing voters who are concerned about illegal immigration could pose an equally important voting bloc.

“This poll shows pretty clearly the challenges that Republicans face, but the challenges for the Democrats are just as tricky,” he said.

A spokesman for the Republican National Committee, Hessy Fernandez, downplayed the study’s findings. “Hispanic voters judge candidates based on where they stand on the issues, and the Republican Party is more closely aligned with Hispanics than liberals like Hillary Clinton,” he said. The telephone survey, which ran through October and the beginning of November, indicates that most Hispanic voters, 41%, support Senator Clinton among the Democratic candidates. Among the Republican candidates, Mayor Giuliani stands out with 35%.

The number of Hispanic voters has gone up since the last election, to 18.2 million from 16.1 million in 2004. Forty percent of Hispanic population is eligible to vote, a much smaller than percentage than other demographic groups. Still, in some of the states that may matter most in the 2008 election, the Hispanic vote could be crucial. A sizable proportion of voters are Hispanic in New Mexico, Florida, Nevada, and Colorado — four of the six states Mr. Bush won by five percentage points or less in the 2004 election.

In all four states, the projected Hispanic share of the votes in 2008 is more than Mr. Bush’s margin of victory in 2004, according to the study.

In New Mexico, where Mr. Bush won by one percentage point, Hispanics are projected to have 27.3% of the state’s votes in 2008. In Nevada, where he won by three percentage points, Hispanic votes are expected to account for 9% of the total. In Florida and Colorado, where Mr. Bush won by five percentage points, they are expected to have as much as 10% of the vote.

Mr. Schnur noted that Senator Kerry might have won the White House in 2004 with those states if the proportion of Hispanics leaning toward the Democrats had been what it was in 2000, and what it is again now.

A former New York congressman who supports a path to citizenship for illegal immigrants, Herman Badillo, said he was not surprised by the poll’s results.

“If they keep this up, they’re going to lose the Hispanic vote and that will guarantee that a Democratic candidate is going to win,” Mr. Badillo, a Republican, said.