Republican Party officials admitted yesterday that their 100-day campaign to switch the registration of 100,000 Democrats in four states fell far short of its goal, but declared the effort "a remarkable success."

A tally released by the party indicated that 54,179 Democrats in four targeted states had reregistered as Republicans; 3,224 had signed cards expressing a desire to switch and 40,636 had pledged on the telephone to switch parties -- a category the GOP labeled "changes in progress" -- for a total of 98,039.

To reach the 100,000 voter goal announced in May, Republicans added 2,410 converts from California to those in the four targeted states of Florida, Louisiana, North Carolina and Pennsylvania.

"From our perspective, Operation Open Door has been a remarkable success," Republican National Chairman Frank J. Fahrenkopf Jr. told a crowded news conference.

But Fahrenkopf, surrounded by cardboard boxes containing names of converts, said a reporter who suggested that the party had failed to meet its specifically announced goal was "100 percent correct. It [that goal] hasn't been achieved. [But] What we are looking at is 100,000 Democrats who have said, 'The Democratic Party no longer represents what I want: I'm calling myself a Republican.' "

"How can anyone realistically say that's a failure?" he asked later. "It's not a failure."

Democrats disagreed, however, calling the GOP figures phony. "The Republicans used the same arithmetic to calculate 'Open Door' results that they used to predict a balanced budget four years ago," said Democratic National Committee Chairman Paul G. Kirk Jr.

The results of the $750,000-program are the second Republican embarrassment in recent weeks. Earlier this month, Republicans failed to pick up an open congressional seat in Texas, despite a $1.5 million campaign and confident predictions.

The Republican goal for its conversion campaign had been to reregister 45,000 converts in Florida, 25,000 in Louisiana and 15,000 each in North Carolina and Pennsylvania. They met the goal only in Louisiana, where the party claimed 29,248 registration converts and 7,184 "changes in progress."

Even counting "changes in progress," the GOP reported only 28,000 switches in Florida, a state where the national party hired solicitors to go door-to-door for $4.50 an hour and offered a $5 bounty for every convert.

Fahrenkopf blamed the Florida showing on hot summer weather and successful 1984 registration drives, which brought in 350,000 new Republican voters.

The GOP reported 7,075 switches in North Carolina.