Senate Minority Leader Howard H. Baker Jr. dropped out of the race for the Republican presidential nomination yesterday, conceded that his campaign "isn't going anywhere."

Returning to the Senate after his fourth-place finish in the Massachusetts and Vermont primaries Tuesday, Baker told reporters that he had done his best, only to find it wasn't enough.

"I'm doing everything I know how do to . . . doing it well . . . and it's still producing third-and sometimes fourth-place finishes," said Baker, who began the race as the leading officeholder in the GOP pack only to become its first dropout since the start of the delegate-selection caucuses and primaries.

Claiming he now wants only to help elect more Republicans to the Senate and become its majority leader, Baker said he has "no aspirations" for the vice presidential nomination, for which he was considered in 1968 and 1976. But he stopped short of saying he would refuse it.

Nor did he rule out another presidential bid in 1984, saying, "It's difficult enough to reach this decision without having to reach a decision about running again."

The 54-year-old, three-term Tennessee senator was joined by his wife and daughter as he hunched over a coffee table before a roomful of reporters in his Capitol Hill office, groping to explain precisely why his campaign never got off the ground.

It was probably a combination of his late start, organizational problems and "mainly because the country hasn't made up its mind what it wants" in a president, said Baker, who had touted himself as a moderate with the best chance of winning in November.

The mood, he said, is "so volatile, so changeable now that I haven't the foggiest idea how this election contest will turn out."

Like the country, baker was in no hurry to line up firmly behind any particular candidate. For the time being, he said, he would not recommend any of his former rivals to his followers -- who include eight delegates to the Republican National Convention, as opposed to 36 for George Bush, 35 for Ronald Reagan, 15 for Rep. John Anderson of Illinois, and one for former Texas governor John B. Connally, according to a United Press International tally.

Baker's return to the Senate was preceded by that of Sen. Bob Dole (R-Kan.), but Dole has not yet announced his withdrawal from the contest. Earlier, Republican Sens. Lowell P. Weicker Jr. of Connecticut and Larry Pressler of South Dakota briefly bowed in and out of the race without mounting major campaigns.

Baker said yesterday he had spent about $4 million and raised $3.5 million, leaving him with a $500,000 campaign debt that he hopes to erase soon.

Baker got a late start, staying in the Senate until November. When the strategic arms limitation treaty (SALT II) got delayed and then shelved, Baker lost a chance to kick off his campaign by leading the opposition to the accord. But even after he launched his campaign, he stumbled badly by bringing a planeload of national reporters to Maine to witness an expected straw-poll victory that backfired when Bush won.

Baker had hoped to do well enough in Arkansas and Puerto Rico to propel him into the front ranks in New Hampshire, but he trailed Reagan in Arkansas and Bush in Puerto Rico. He wound up third in the Iowa precinct caucuses and third in the New Hampshire primary, with about 13 percent of the vote. Baker won 5 percent of the vote in Massachusetts and 13 percent in Vermont.

One irony of the Baker campaign's demise was that some of President Carter's political strategists considered him potentially the most formidable of the GOP contestants for November. They cited his political skills and said he projected a broader appeal for a general election than any of the other Republican aspirants.

But Baker was aiming at a narrower constituency in the GOP caucuuses and primaries, and competing with Bush, Anderson and others for the moderate Republican vote.

One lesson of the campaign may be that a candidate has to be unemployed and run full time for two years or more, Baker suggested. But he quickly added, "If that's the way it is, that's not the way it ought to be."

As for his brief campaign interlude, he said, there were some hotels he wouldn't miss, but "it is an absolutely worthwhile experience . . . an experience I will cherish for the rest of my life."