The Virginia campaign of Ronald Reagan, already considered the favorite presidential candidate of the state's rank and file Republicans, got a decisive boost today when all three of the GOP's statewide office holders endorsed the former California governor.

Gov. John N. Dalton, Sen. John W. Warner and Attorney General J. Marshall Coleman issued a joint statement praising Reagan as "a proven leader who offers the ability, strength and vision we need to put our nation back on track."

The endorsement was a particularly sweet one for Reagan because both Dalton and Coleman had supported Gerald Ford four years ago, and many observers had expected one or all of them to back an alternate candidate to Reagan this year. As recently as two weeks, Dalton and Warner had issued a joint statement of political neutrality, but a series of rapid-fire events since then appear to have tipped the political balance decisively in Regan's favor.

One day after the statement, Ford announced his refusal to run. Then Reagan won crushing primary victories in several Southern states, as well as in Illinois and New York.

Reagan got another show of support tonight at the state GOP's annual Commonwealth Dinner, where 198, nearly two-thirds of the 310 diners, voted for him in an informal straw poll. George Bush was a distant second with 62 voters.

In their statement today, Dalton, Warner and Coleman said that "in primary after primary," Reagan has "shown an appeal that cuts across party lines."

"It's a clear symbol to any observer that Virginia has made up her mind and it's Reagan,' said a jubilant John Alderson, Reagan's state coordinator, who predicted the former governor's campaign would exceed its original goal of capturing 44 of the state's 51 delegates to the Republican convention (to be held in Richmond June 6-7) with a solid front," said Alderson.

Four years ago, Reagan took 35 of the state's delegates despite the fact that then Gov. Mills E. Godwin and Dalton, then the state's lieutenant governor, supported Gerald Ford. This year, the Reagan campaign had expected its strongest challenge from George Bush, but the successive Reagan primary triumphs appeared to have all but crippled the state Bush effort expect for pockets of support in Northern Virginia, Tidewater and Richmond.

"As I read it, Bush is just hanging on," said Dalton press secretary Paul G. Edwards. "He's a very, very distant second. I don't think anybody believes he's viable."

U. S. Rep. G. Willian Whitehurst of Norfolk, Bush's Virginia co-chairman, said the Bush campaign has no intention of writing off Virginia. c

"It's not over," said Whitehurst, who noted Bush had shown some strength in early GOP mass meetings in Alexandria and some parts of the state. "We'll go in and battle for out share and hope we can pull some out."

Virginia Republicans are picking their delegates to the state GOP convention in a series of mass meetings that began earlier his month and will continue through early May.

As recently as 2-1/i weeks ago, Dalton had spoken glowingly of Ford at a press conference, pointing to polls showing the former president as a stronger national candidate against President Carter than Reagan. Dalton backed off from a full endorsement and later declared his neutrality. Edwards said events since then had made Reagan's nomination appear all but inevitable.

"Reagan has clearly solidified the situation in the past two weeks," said Edwards.

Edwards said Warner and Dalton discussed the endorsement two days ago, when Dalton called Coleman, who previously had stayed completely silent on the contest. The statement was issued today just hours before the state GOP's annual Commonwealth Dinenr here.

Aldeson predicted Reagan would have little trouble beating either Carter or Sen. Edward M. Kenedy in November in the state. Virginia was the only southern state in which Carter lost to Ford four years ago.

The GOP dinner was a rousing partisan affair as Republicans honored Sen. Warner and six GOP congressmen from Virginia. Republicans hoped to capture at least one more of the state's four Democratic congressional seats this fall. They also are confident they can deliver Virginia to the GOP presidential nominee, as they did in 1976.

They made it clear that the chinks in Carter's record as president -- including high inflation and interest rates, the Iranian cricis and the Soviet invasion of Afghanistan -- will be big GOP issues if he is renominated.

"That's not the kind of a record you run on, it's the kind you run from," said Whitehurst in one of tonight's speeches. "Carter can't leave Washington because of a D.C. ordinance saying that you can't leave the scene of an accident."