Sen. Howard H. Baker Jr. (R-Tenn.) came here today expecting to get his presidential campaign off to a fast start with a straw vote victory at a big Republican forum, but instead was tripped up by George Bush.

Baker's people thought they had the victory all set up, because Sen. William cohen (R-Maine), the state's senior Republican, had made an all-out effort to line up support for Baker with the political apparatus that sent Cohen to the Senate last year. Bush has a summer home in the state but has not campaigned here extensively.

Baker's timing of his announcement as a candidate Thursday was keyed to the two-day trip through New England that was to be climaxed by a victory here at the Republican forum, where all presedential candidates except Ronald Regan made a personal appeal for support.

But when the votes were counted, Bush had received 466 (34.8 percent) to Baker's 446 (33.3 percent). Former Texas governor John Connally was third with 236 votes, forllowed by Regan with 98 and Rep. Philip M. Crane (R-Ill.) with 62. Sen. Bob Dole (R-Kan.) received 9 votes and Rep. John Anderson (R-Ill.) 6.

Although the voting was not binding, the outcome was important to Bush It could launch his campaign from nowhere, as a fast early start in Iwoa did for Jimmy Carter four years ago.

Bush, in a statement after the vote, said he was delighted to have won, "particularly since Sen. Baker had the support of an extremely popular U.S. senator, Bill Cohen, who put his organization totally behind his effort."

The former Central Intelligence Agency chief said the victory showed he had strong organizational support in more states than Iowa, "and it is a credit to the outstanding people who are backing my candidacy in Maine."

"Obviously we were more than pleased with the results," said David Keene, Bush's campaign director in Washington. "We did not expect to win."

Keene said that as of Friday, it appeared that Baker had about 25 percent of the delegates; that Reagan Connally and Bush were grouped with 10-15 percent each, and 30 percent were undecided.

He said Bush appeared to have picked up most of the undecided delegates and attributed that to organizational strength and Bush's performance in Maine today. "Our people said Bush's performance was very good and that Baker's was lackluster," Keene said.

Then, savoring the surprising victory, he added: "Remember all that stuff we were saying earlier in the week about this not being important? We were wrong."

Baker, obviously disappointed, told a large press contingent that he had encourged to accompany him to witness his expected victory, that "I face the same problem I've faced from the beginning, which is that at some point I have to move out of second place into number one. At some point I am going to be number one."

Baker said he thought it significant that he, and Bush, who will be competing for the moderate-conservative vote, ran first and second while Connally, Regan and the rest ran well behind. Bush showed much more strength than most had expected, while Connally and Regan showed considerably less, Baker said.

"It shows there is more of a moderate vote than a lot of people thought," Baker said. "It probably increases the likelihood that Bush and I will be the main contenders in New Hampshire," where the nation's first primary will be held in February.

The gathering of delegates to a "Maine Republican presidental forum" put on by the state party organization has no official status. The state convention that will send delegates to the national presidential nominating convention will be held in Arpil. Delegates to the state convention will be selected by town caucuses, starting in January.

But the delegates to today's forum also were elected by town caucuses. Republican State Chairman Hattie Bickman said she believed 90 percent of the people who will be delegates to the April state convention were at today's meeting. If so, today's lineup should closely resemble the 21-member delegation that Maine Republicans will send to the national convention in Detroit to select their presidential nominee.

The other candidates had conceded this vote to Baker in advance of the powerful lift he received from Cohen. Bush had dismissed the straw vote in advance as a "beauty contest" that doesn't means anything. The first meaningful events on the road to Detroit will be the local caucuses in Iowa that start the delegate-selection process, Bush has said. Bush has a strong organization in Iowa and hopes to do well there.

On his two-day trip through five New England states, Baker was greeted warmly Friday morning by students at Brown University in Providence, R.I., and later told a noon gathering at Quincy Market in Boston, home of Sen. Edward M. Kennedy (D-Mass.), that he would say nothing about others' private lives. Some considered this a gratuitous reference to Chappaquiddick, which Baker has said he would not talk about.

At Concord, N.H., Friday evening a crowd estimated at more than 1,000 turned out at the National Guard Armory for a free barbecue, which Baker had flown up from Tennessee. This morning he flew to Burlington, Vt., and received very close to an esdorsement from Republican Gov. Richard Snelling, although Snelling is still involved in a not-yet-dismantled draft-Ford movement.