Ronald Reagan has decided against selecting his first choice for a vice presidential running mate, Sen. Paul Laxalt of Nevada, on grounds of regionalism and party unity.

Elimination of the conservative Laxalt, a close friend of Reagan, swung the early odds at the Republican National Convention here toward one of three relatively moderate candidates -- former ambassador George Bush, former secretry of defense Donald H. Rumsfeld or Michigan Rep. Guy Vander Jagt. All have close ties to former president Gerald Ford, whose endorsement of the vice presidential choice is a criterion in the decision-making.

The only other propsective running mates who are considered to have any real chance are Indiana Sen. Richard G. Lugar and New York Rep. Jack Kemp, both of whom have conservative backing at the convention.

Laxalt, who won Reagan's gratitude for coming to his help at the low point in the 1976 primaries, was the former California governor's undisputed first choice for the No. 2 spot.

But Reagan, a political realist who anticipates a close contest with President Carter, is said to have rejected Laxalt on pragmatic grounds. Laxalt comes from neighboring Nevada with a throwaway three electoral votes thought to be in the bag for Reagan.

"In many ways, Laxalt is like a younger Reagan and from the same region," says one who knows both men well. "He brings nothing to the ticket that isn't already there."

With Laxalt out, Reagan is heading into the conservative cauldron of the Republican National Convention without having made up his mind and without having interviewed the top candidates.

In a brief interview last week in Los Angeles, Reagan said he hoped his choice, when it comes, would contribute to party unification, and "that even those who have another choice would accept it."

But there are moderates in the Reagan camp and beyond who fear that the Republican convention is the worst place to arrive at such a choice.

"The convention is far to the right of Gov. Reagan," says a moderate who favors Bush. "It's easy to forget, in the passions of the moment, that the Republican convention is far from mirroring the country."

While it may be logical to assume that the convention is ripe for compromise after conservative platform victories against the Equal Rights Amendment and federal funding of abortion, there is a belief within the party's moderate wing that this has merely whetted the appetite of the victors. p

The main moderate hope is that conservative passion on the vice presidency is directed more against Sen. Howard H. Baker Jr. of Tennessee than it is for any other candidate.

Bush's views are not that far from Reagan's on most issues (ERA, which Bush favors, is an exception), and Reagan is known to consider Bush ideologically compatible.

Reagan also is said to realize that Bush would help unify the party, bringing in such genuine moderates as Michigan Gov. William Milliken, whose strenuous campaigning led the way to a big Bush victory over Reagan in the May 20 Michigan primary. And there is gruding admiration in the Reagan camp for the way Bush fought on in the presidential race after losing a string of primaries.

However, Reagan believes he saw signs of weakness in Bush at their celebrated Feb. 23 debate in Nashua, N.H., where an angry Reagan grabbed the microphone and, some would say, an edge in the Republican primaries.

Bush's political paralysis at Nashua has not been forgotten, and it could prove decisive when the moment for decision comes.

Though Reagan asked for, and received, a full file on former ambassador Anne Armstrong of Texas and is known to admire her, she is considered out of the running. She has been offered a top campaign job, but the announcement has been withheld lest it seem an advertisement of vice presidential rejection.

Believed virtually out of the race are three persons who have been considered extensively by Reagan and his staff -- Baker, because of the right wing oppostion; former treasury secretary William Simon, because he has never run for elective office, and Ford, because he doesn't want to be considered.

Reagan and his staff are mindful, however, of what one of them calls "the Ford factor." Ford has been making two standard speeches around the country, one of them blasting Carter and the other predicting that the general election will be decided by the House. The Reagan forces would like a little more from Ford, and one of the ways to get it may be to pick someone favorable to him.

This probably means one of three people -- Bush, who inherited most of Ford's 1976 campaign staff; Rumsfeld, who was Ford's top aide, or Vander Jagt, also from Michigan.

Bush, 56, is a favorite of the poltical people around Reagan because he brings, as one of them puts it, "instant unity" to the party. He is also widely perceived as a hard worker and he has ties to the Northeast. It is thought he would help with ticket-splitting voters and moderate Republicans but, on the basis of his primary record, he is believed to have little reach with the working-class Democrats whom Reagan wants to target in the general election.

Rumsfeld, 48, has risen swiftly from the status of darkhorse possibility to near front-runner. He would bring a lot of pluses -- the Ford connection, plus experience as ambassador to NATO, congressman from Illinois, a key state and executive action in the White House.

Now the head of the pharmaceutical firm of Searle & Co. in Skokie, Ill., Rumsfeld could help himself by shining at an early convention appearance. Of all the candidates, Reagan knows Rumsfeld the least, which is not necessarily a minus. Reagan chose then-Rep. Ed Reinecke to run for lieutenant governor of California after only a few conversations.

Kemp, 45, may have been hurt by an organized conservative campaign in his behalf, even though he quickly quashed it. He is distrusted by some members of the Reagan staff who view him as too brash and ambitious, but he has solid support with the grass-roots corps of Reagan's regional political directors, who prefer Kemp to anyone.

As the architect of the Kemp-Roth tax-cut plan, the centerpiece of Reagan's domestic advocacy, Kemp is seen by some as upstaging the presidential candidate on this issue. Kemp is an effective but controversial campaigner -- "high risk, high gain," in the view of one Reagan intimate.

Lugar, 48, a one-time Rhodes scholar with a conservative voting record in Congress, is held in high regard by Reagan. "He has no negatives," says one observer. But, according to the survey taken for Reagan by pollster Richard B. Wirthlin, Lugar also has little name recognition outside Indiana. "Most people think Lugar is a pistol, not a senator," is a recurrent joke. But if Reagan decides to make a tough antiabortion stand a test, and thereby rules out Bush, Lugar could become the compromise.

Vander Jagt, the 48-year-old-former Yale divinity student who has been a congressman since 1967, is not taken seriously by some Reaganites. But he is by Reagan, who likes him and who is aware that Vander Jagt's voting record has become more conservative since 1975.