In Nassau County, an Influx of Democrats Threatens a G.O.P. Stronghold HEMPSTEAD, N.Y. - With a pivotal local election looming in the fall, the Republican-controlled Nassau County Legislature recently approved new district lines that could help the party secure at least two more seats. A local Republican club has increased its campaign chest tenfold over the last two years, bringing in individual contributions of tens of thousands of dollars from a family paving company, a real estate developer and other donors with business before the county, according to disclosure records. And if past elections are any guide, the party can also turn for campaign help to workers from the county's special tax districts, which handle services like water and sewage. Most are run by loyal commissioners. The party is marshaling its forces because it is facing an unprecedented challenge in the postwar era: a deluge of Democrats. In a county that has long symbolized Republican dominance of the American suburbs, Democrats now make up a majority of party-affiliated voters. Last year, county residents registered to vote at a rate of two to one in favor of Democrats, according to the county's Board of Elections. The changes reflect a liberal swing in some suburbs around the country as more young people, immigrants and minorities move in. "For the Republicans, the demographic shift is truly existential," said Lawrence C. Levy, an expert on Long Island politics at Hofstra University. "It's the difference between life as they know it and the death of the party as it is," Mr. Levy said. "There will be so many more Democrats crowding into formerly Republican precincts that the G.O.P."s ability to gerrymander its way out of trouble will become more and more remote." Republicans in Nassau had a preview of this challenge in 1999, when they lost control of the County Legislature after a series of scandals, and then the county executive's office two years later. But they recovered. The county executive is now a Republican, and the party controls the County Legislature, 10 to 9 , as well as all five State Senate seats in Nassau. The county executive, Edward P. Mangano, declined to be interviewed, but his office and other Republican leaders said the party had regained control because its message and policies transcended partisan divisions. "In fact, traditionally Democratic populations have received improved services under this administration than the last," Mr. Mangano's office said in a statement. Democratic leaders accused Republicans of maintaining power through gerrymandering, intimidation of Democratic voters at polling places and campaign finance improprieties. There was a time when the Republican Party in Nassau County was a nearly indomitable force, a political machine with influence that extended to Washington. Ronald Reagan is said to have likened the county to heaven on earth for his ideological brethren. The party's power was embodied by Alfonse M. D'Amato, who was a local official in Hempstead before becoming a United States senator. He is now a lobbyist. Only two Democratic county executives have been elected since the position was created in 1938. But about two decades ago, the demographics began to shift. The proportion of minorities in Nassau County has grown to 22 percent from 14 percent since 1990, according to census figures. Many of the newcomers are from Brooklyn and Queens, with a Democratic bent. Compared with a decade ago, there are nearly 82,000 more registered Democrats in Nassau County, while there are more than 16,000 fewer Republicans, according to the State Board of Elections. As of April, there were 371,557 registered Democrats in the county, and 332,036 registered Republicans. Political analysts said the question now is whether the Republican Party is so entrenched that Democrats will have difficulty dislodging it. With the lines redrawn, Republicans have gained a significant edge in two districts in the County Legislature where Democrats previously were the majority. The new map also bolsters the Republican advantage in a third district.