Fairfax Republican Rep. Stan Parris today became the first candidate to formally announce for the 1985 race for governor of Virginia, boasting of a campaign treasury so large that it surprised many Republicans and seems certain to make him a serious contender for the party's nomination.

Parris, 55, is expected to face two better-known GOP rivals in the race but he announced today that his campaign is beginning with $301,000 in the bank -- more than twice the combined total his two opponents have raised. The member of Congress, who won his fourth term in the House this month by his largest margin ever, has invested $100,000 of his own money in the race.

Parris' announcement marked the beginning of his effort to overtake former state legislator Wyatt B. Durrette of Richmond, considered the early front-runner, and J. Marshall Coleman of McLean, a former state attorney general who lost the governor's race to Democrat Charles S. Robb in 1981. Gov. Robb cannot succeed himself under Virginia law.

Durrette, 46, who represented Fairfax in the General Assembly, is scheduled to formally announce next week, and Coleman, 42, who is practicing law in Washington, is deciding whether to run for the top office or lieutenant governor.

Parris' entry is likely to make for a costly race that could go all the way to the party's state nominating convention on May 31 in Norfolk. It could collapse, however, within months if neither Parris nor Coleman can shake Durrette's acknowledged lead, according to campaign aides.

The developing campaign, the first in several elections to have more than one strong GOP candidate for governor, also is unique because all three candidates have close ties to Northern Virginia, an area once shunned by many Virginia politicians. They considered it too closely identified with Washington and too difficult and costly for their campaigns.

As expected, Parris today stressed his conservative credentials and "electability," an indirect slap at his opponents. Durrette lost a 1977 nomination bid for state attorney general to Coleman and the 1981 attorney general election to Democrat Gerald L. Baliles when Coleman lost to Robb.

"We must nominate a candidate who is most likely to win in November, someone who has a strong vote ase, someone who can raise the funds necessary to wage a successful campaign," Parris said.

A conservative, he is a former Fairfax County supervisor and three-term member of the General Assembly. He is not widely known outside his 8th Congressional District, which includes Alexandria, southern Fairfax, eastern Prince William and northern Stafford counties.

Parris, who first mentioned his interest in the governor's office years ago, said he was concerned that some constituents might be angry that he jumped into another race so soon after his reelection to Congress.

"I'm not going to leave the 8th District as such," he said today. "I'm going to broaden the base . . . .I think that people in my congressional district generally. . . support my doing so."

Parris said he expected to spend at least $600,000 in trying to secure the nomination. Most of today's contributions came from Fairfax and Alexandria businessmen, including Fairfax developers John T. (Til) and William T. Hazel, two brothers who gave a total of $50,000.

The coming weeks are crucial as the candidates line up support for party "mass meetings" that will be held as early as February to begin selecting delegates to the nominating convention. Already, sophisticated direct mail computers, which helped Parris score a convincing reelection victory Nov. 6, are cranking out thousands of letters to the potential delegates.

"He's got a shot at it," said University of Virginia professor Larry Sabato, a longtime observer of Virginia politics.

Sabato said Parris is entering "with the resources to run a Cadillac campaign, but he still has to be rated third and that the convention will be "Durrette's to lose."

The Democrats are enmeshed in their own three-way campaign for governor with Baliles, Lt. Gov. Richard J. Davis and Hampton legislator Richard M. Bagley.

Republicans are anxious to avoid a repeat of 1981 when Robb, Davis and Baliles swept all-three top offices. "We're still stinging from the defeat we suffered . . . .we're determined that that will not happen again," said John E. Alderson Jr. of Roanoke, a cochairman of Parris' campaign. "This party needs a winner."

Richard Cullen, a Richmond lawyer close to former Republican governor John N. Dalton, who declined to run again, said Parris' campaign makes an unpredictable three-way race.

"Strange things happen after the first ballot at Republican conventions in Virginia," Cullen said.

Despite continuing pressure and warnings of party divisiveness among the three campaigns, Cullen said Dalton, who could have had the nomination for the asking, has firmly ruled out any decision to run again. CAPTION: Picture 1, REP. STAN PARRIS . . . campaign has $301,000 in bank; Picture 2, REP. STAN PARRIS . . . expects to spend $600,000; Chart, MAJOR INITIAL CONTRIBUTORS TO PARRIS CAMPAIGN