Senate Republican Leader Howard H. Baker Jr., once a leading contender for the 1980 GOP nomination, today endorsed Ronald Reagan for president, saying, "The time has come to unite our party."

"Only divisions within our party can keep us from benefiting from the bitter divisions within the Democratic Party," Baker said.

"I think the time has come for our party to rally around our nominee," he said. "And it's obvious that Ronald Reagan is our party's nominee."

One of the party's leading moderate spokesmen, Baker is the fourth Republican contender to endorse Reagan after dropping out of the race.

The endorsement came two days before the Pennsylvania primary in which George Bush, Reagan's chief remaining rival for the Republican nomination, has waged a last-ditch effort to overtake the former California governor.

It represented a continuation of the coalescing of the Republican establishment around the candidacy of Reagan, once thought to be too conservative to capture the nomination.

"Having sought the nomination myself, I have a special firsthand appreciation of the breadth and depth of Gov. Reagan's personal appeal," Baker said in this Reagan stronghold in suburban Philadelphia.

Baker announced his candidacy for the GOP nomination in November 1979 but dropped out last March after the Massachusetts and Vermont primaries. Other Republican dropouts who have endorsed Reagan are John B. Connally, Sen. Bob Dole (Kan.) and Rep. Philip M. Crane (Ill.).

Today's endorsement came during a week in which Reagan picked up pledges of support from 36 members of Congress, 17 of the 21 county Republican chairmen in New Jersey, Delaware Gov. Pierre S. du Pont and Ohio Gov. James Rhodes. The endorsements have understandably buoyed hopes among Reagan's traveling entourage, where there is increasing talk about having the nomination locked up long before the Detroit convention in July. "We would not want to take anything for granted, but we are getting closer to a sure thing every day," said press secretary Ed Gray.

The endorsements could be devastating to Bush in the Pennsylvania primary, the first contest that pits the former ambassador and CIA director directly against Reagan.

Meanwhile, Bush picked up an endoresement from Penn State football coach Joe Paterno.

"We think Paterno will bring more votes than Baker in Pennsylvania," Bush press secretary Peter Teeley said.

Earlier, in an interview on "Meet the Press" (NBC, WRC), Bush was asked about Gov. Rhodes' backing of Reagan.

"These endorsements change and change fast, if the perception of being able to win happens," Bush replied. "And that's what I'm counting on and that's what I'm working hard to do, to change these perceptions."

To have any hope of overtaking Reagan, Bush needs a victory here. And even if he should score an upset, the endorsements indicate that he would have a difficult time in later primaries.

Baker left a sickbed to meet Reagan at the Philadelphia airport this afternoon, and later traveled to a rally at the Delaware County municipal building where his endorsement was announced.

Baker praised Bush but said Reagan would make a much better GOP nominee and a better president.

"This country desperately needs a president with a mandate, a clear-cut alternative," Baker told a news conference. ". . . The country is hungry for a clear statement of common sense."

Baker, who is frequently mentioned as a possible vice presidential candidate on a Reagan ticket, was questioned about the matter today. He said the subject had not been discussed between the two men and it would be inappropriate to do so. But both he and Reagan said they had compatible political philosophies that would make it possible for them to work together.

Baker will accompany Reagan on a campaign swing in Pennsylvania on Monday.

Earlier in the day, Reagan was endorsed by Rhodes at a lawn party at the governor's mansion in Columbus.

It was one of those bits of political theater that candidates delight in, an event more of ceremony than substance. Rhodes invited more than 500 of the movers and shakers of the Ohio Republican Party to attend to let them know that after Flirting with illfated candidacies of Connally and Gerald R. Ford he had decided Reagan was really his man.

"This is the Sabbath," Rhodes said. "I am like the Lord; I love everyone. I love George Bush. I love Gerald Ford. I love Ronald Reagan. Sometimes in love you have to make your choice. My choice is Ronald Reagan."

Rhodes is a political survivor of the old school. He believes in votes, not ideology. Until the last few days, he had never shown any particular liking for Reagan. In 1975, he endorsed then-president Ford and helped him narrowly defeat Reagan in the Ohio primary a year later.

But today he declared, "It is vital that our party begin now to unite and join ranks to put a new leadership in the White House."

Rhodes was even blunter when he took Reagan to meet with Ohio county GOP leaders behind closed doors. Reagan assumed Rhodes wanted him to woo the chairmen. "Let's establish some dialogue," one person present quoted Reagan as saying.

But Rhodes promptly cut him off. "Hell with that," the Ohio governor said. "They know what they're supposed to do."

Regarding the Pennsylvania primary Tuesday, Reagan has previously downplayed his chances for winning the beauty contest portion of it, saying his forces were concentrating on the separate delegate portion of the ballot that they were confident of winning.

But today he was optimistic. "I'm vain," he told a news conference in Columbus. "I'd like to win the beauty contest and I'd think this endorsement will help."