The Republican Party, bolstered by favorable poll data and the defection of several prominent Democrats, yesterday announced a $500,000 program to persuade 100,000 registered Democrats to convert to the GOP in four key states.

"Recent polls clearly reflect that for the first time in almost 50 years, as many Americans identify today with the Republican Party as do with the Democrats," Republican National Chairman Frank J. Fahrenkopf Jr. said. "The Democratic Party has lost touch with America."

The program, which Fahrenkopf dubbed "Opereration Open Door," has a stated goal of converting 45,000 Democrats in Florida, 25,000 in Louisiana and 15,000 in both North Carolina and Pennsylvania over the next 100 days.

The states were selected because each has a Senate contest in 1986 and because voters register by party, making it possible to track program results. In addition, recent registration trends in three of the states -- Florida, Louisiana and North Carolina -- have favored the GOP.

Last week, former House member Kent R. Hance (D-Tex.) announced that he had joined the Republican Party. Today, William Lucas, county executive of Wayne County, Mich., which encompasses Detroit, is expected to announce his conversion to the GOP. Both Hance and Lucas are considered likely to seek the Republican gubernatorial nominations in their states. Republicans are particularly pleased about the conversion of Lucas, who is black, because they believe that it will attract black votes.

Operation Open Door was immediately attacked in a statement issued by Paul G. Kirk Jr., chairman of the Democratic National Committee.

"This Republican announcement is a transparent public relations blitz to attempt to salvage the remnants of an opportunity that is steadily going down the drain," the statement said. "It won't work. Political opportunists who change their stripes will find themselves caught in a revolving door. . . .

"As Ronald Reagan's approval rating flips and the Republican Party continues its rightward and divisive drift under the guidance of Jesse Helms and Jerry Falwell, we Democrats will continue to build and broaden our base among those who count the voters."

Poll data shows that the GOP has made significant gains in terms of voter identification over the long haul, although more recent, short-term trends have been in favor of the Democrats.

Washington Post-ABC News polls show, for example, that from November 1979 to March 1985 the Democratic edge over Republicans fell from 28 points, 59 to 31 percent, to 10 points, 52 to 42 percent.

From January to March, however, the Democrats came from 1 percentage point behind to 10 percentage points ahead of the Republicans. A less-detailed poll April 22 suggests that the trend favoring the Democrats is continuing.

The Gallup Poll found that for the entire first quarter of this year, the Republican Party had achieved near-parity with the Democratic Party, with the GOP trailing by 2 percentage points. A spokesman for the Gallup organization said, however, that four polls were conducted in the first quarter, and that Democrats appeared to be gaining strength by March.

In the past 20 years, the GOP has succeeded in sharply reducing what had been a consistent 18- to 22-point Democratic advantage. Richard B. Wirthlin, President Reagan's pollster, said yesterday that the Republican gain "is the single most important political development in the last decade."