Paul S. Trible Jr.'s surprise decision to leave the U.S. Senate after one term has the immediate effect of confounding a Republican Party desperate for political gains in Virginia by all but handing the seat to the Democrat who has been most coy about seeking it.

Yet, if two things were instantly clear after Trible's announcement -- Republicans were flabbergasted and Democrats salivated anew about the prospects of former governor Charles S. Robb running for that open seat -- Trible's real reasons for retiring and his plans remained considerably more murky.

In his brief, televised address, a relaxed Trible listed only two reasons for leaving the Senate: the "very personal" one to spend more time with his family and his growing "frustration" with a workload of Senate hearings and debates that seem to "go on and on and on."

At the same time, Trible tantalized Republicans and Democrats alike with a broad hint about seeking another office, one that regulars in both parties speculated would be that of governor. "I'll seek other opportunities to make a difference in the life of our state," Trible said.

Several senior Republican strategists said last night that while they did not dispute Trible's sincerity about his family commitments, there were other, more pressing political considerations, including the possibility of a Robb challenge, that also were a factor in his decision.

"This stuff about the family is very sincere -- I know Paul means that," said one GOP strategist. "But let's be honest. This way he can go out a winner and spend his $1.5 million on the governor's race.

"He's telegraphed that tonight," this party official went on. "I can't imagine a 40-year-old guy who's never lost an election giving up on politics. And Paul Trible loves politics."

However, within hours of Trible's announcement, some Democrats were already scoffing at the thought of the state's junior senator running for governor in 1989.

"If you take Senator Trible at his word, it's hard to imagine the Republican Party would nominate him for a job that's at least as demanding as the one he is leaving," said Chris Bridge, press secretary to Democratic Gov. Gerald L. Baliles.

But several Republicans, especially those who are cool to the all-but-announced candidacy of former state attorney general J. Marshall Coleman, say Trible would be a strong contender for capturing the governor's mansion two years from now.

Many political observers are far less optimistic, though, about the GOP's chances of retaining Trible's seat next year. While there likely will be no shortage of candidates for the nomination -- Rep. Stan Parris of Fairfax County among them, perhaps -- none would enjoy Robb's enormous popularity in such a statewide race.

"I think this means the Republicans will lose this seat," said Thomas R. Morris, a University of Richmond political scientist. "For Chuck Robb, this would be a 'gimme' seat. It would be his for the taking."

Trible said in an interview last night that there is "a whole host" of Republicans who could win his seat next year. He added that those seeking the job might want to take a cue from the then-obscure congressman named Paul Trible who laid an early claim on Harry F. Byrd Jr.'s Senate seat five years ago.

Trible said that he made last night's announcement "very early on so that the Republican Party will have time to pick an able candidate." Having done that, he added, "It's a whole new ball game for the Republicans and Democrats."