New Jersey Gov. Christine Todd Whitman (R) unexpectedly announced yesterday that she will not run for the Senate next year, encouraging Democrats and depriving the GOP of one its top prospects for 2000.

Whitman announced her decision outside her State House office in Trenton, saying she wants to focus on the two and a half years remaining in her term instead of being distracted by the rigors of what promised to be a long, grueling and expensive race.

"I have determined that I can't be the kind of governor I want to be and give the Senate campaign the kind of attention that it needs," Whitman said.

Republicans were counting on the popular second-term governor to provide an insurance policy for their 55-45 Senate majority by winning a seat currently held by a Democrat. Overall, Republicans must defend 19 Senate seats next year, compared with only 14 for the Democrats. Several of those 19 seats are held by vulnerable freshman senators elected in the GOP landslide of 1994.

Republican officials in New Jersey and Washington expressed surprise that Whitman, who had raised more than $2 million for the race and led by large margins in early polls, chose to skip the contest for the seat of retiring Sen. Frank R. Lautenberg. The seat had been considered among the Democrats' most vulnerable.

Whitman's departure leaves a gaping hole in the Republican field, which now consists solely of former Libertarian candidate Murray Sabrin. But GOP officials in both New Jersey and Washington yesterday mentioned several possible replacements, including Reps. Frank LoBiondo and Bob Franks, state Sen. Bill Gormley, Essex County Executive Jim Treffinger and Jersey City Mayor Bret Schundler. Franks announced yesterday that he is creating an exploratory committee.

"Republicans went from having a candidate who is known and liked to having [an eventual nominee] who is unknown, untested and may have to go through a divisive primary," said New Jersey political analyst Cliff Zukin, director of the Star Ledger-Eagleton Poll at Rutgers University. "It's really a terrific day for the Democrats in New Jersey and a pretty bad day for the Republicans."

Democrats dismissed talk of other Republicans emerging and portrayed the race as a virtual lock. "The eventual Republican nominee is now crippled," said Sen. Robert G. Torricelli (N.J.), chairman of the Democratic Senatorial Campaign Committee. "Raising $12 million in 12 months is not going to be possible for the Republican candidates who are being discussed. This one is ours."

Democrats may face a bruising primary of their own, however. Former governor Jim Florio, who lost to Whitman in 1993, is considered the front-runner but faces a stiff challenge from Jon Corzine, former chairman of the New York investment bank Goldman, Sachs & Co. Corzine has said he will spend as much of his money as it takes to win. The Democratic field also includes former party chairman Tom Byrne.

CAPTION: New Jersey Gov. Christine Todd Whitman's decision gives Democrats an unexpected break.