Virginia Republicans, stunned by their first major statewide defeat in 16 years, turned on their fallen gubernatorial candidate and on each other today as they groped for an explanation for yesterday's Democratic sweep.

J. Marshall Coleman, whose brash, self-confident style had grated on many GOP elders, was roundly criticized for running a campaign for governor that relied too heavily on President Reagan's coattails and too little on the advice of fellow state Republicans. Many argued that the GOP campaign wrote off large chunks of the electorate, including the state's black voters, without galvanizing conservatives in the traditionally Republican suburbs.

"If the party has a failing, it was a failure to communicate to the central cities," conceded state GOP Chairman Alfred Cramer. "We may have depended too much on suburbia . . . ."

Even Gov. John N. Dalton, Coleman's most ardent supporter during the campaign, sniped at his political protege at a press conference today, calling Coleman's pledge to veto any tax increase "a mistake. . . I felt the tax issue would come home to haunt him."

Others insisted it wasn't so much Coleman but his party, overconfident after more than a decade of electoral victories, that was to blame. "The Republicans finally got so cocky and arrogant that they brought the Democrats back together," said one GOP operative. He derided efforts by the Republican National Committee to salvage Coleman in the campaign's final weeks as "that super-jock Republican stuff" that turned off Virginia voters.

Robb overwhelmed Coleman by 54 to 46 percent, accumulating a 105,000 vote margin, and winning nine of the state's 10 congressional districts, thanks in large part to an unexpectedly high voter turnout of more than 65 percent. Democrats and Republicans alike credited Robb with forging a remarkable coalition of old-line conservatives and traditional Democrats such as blacks and liberals.

Both sides cited a number of factors in the heavy turnout, including warm, sunny weather and the most elaborate Democratic get-out-the-vote effort in recent years. But the key element, both sides agreed, was Coleman's constant emphasis on the Reagan connection, a strategy even the Republicans admitted had backfired.

Republican state Sen. Wiley F. Mitchell of Alexandria said the vote in his Northern Virginia district was close to 20 percent higher than he expected in a gubernatorial year. Mitchell said blacks, lower-income and elderly voters came to the polls to send a message to the White House. "They were not particularly interested in candidates, issues or experience; they were interested in sending a message across the Potomac River."

In Richmond, by the estimate of Robb operative Paul Goldman, blacks turned out at a record 63 percent and gave Robb a huge 97 percent of their votes. "There's a lot of fear of Reagan and his policies," said a jubilant Richmond Mayor Henry L. Marsh, a black Democrat.

Ardent Reagan supporters offered a different explanation. Reagan's 1980 Virginia campaign manager, John Alderson, said Coleman, who won the state attorney general's office running as a progressive Republican, was simply unconvincing in Reaganite garb.

"It borders on ludicrous to say he's running as a Reaganite and not tap anyone in the Reagan organization to advise the campaign," said Alderson.

Coleman was not available to answer his critics. He spent the day behind closed doors and tightly shuttered windows at his home in Richmond, refusing to come out to talk to reporters.

"He just wants his privacy today," said his sister-in-law, Mary Fox, in front of the house, which in recent weeks had been decorated with several Coleman posters. Today, the posters had vanished.

Anson Franklin, Coleman's campaign manager, said his man had lost because he "had the misfortune to run against the best candidate the Democrats have ever had." Franklin said a key element was that Robb, with his dark hair and deep voice, appeared more "gubernatorial" to television viewers, a fact he said was confirmed by the campaign's polls.

"It's no knock on Chuck Robb to say he's straight out of central casting and he could create his record from scratch because he didn't have one," said Franklin.

Other Republicans also pointed at state Sen. Nathan H. Miller, Coleman's running mate for lieutenant governor, who lost badly in his race after being accused of conflict of interest. "Nathan received a mortal blow and this did have a downward effect on the ticket ," said Cramer.

Still others suggested Coleman was doomed since the GOP nominating convention at Virginia Beach turned its back on party elders last June to nominate Miller over state Sen. Herbert Bateman. They said Miller's nomination gave the party a ticket of three young, aggressively partisan Republicans, who had little appeal to the conservative independents the GOP has relied on in its victories.

"They made the classic mistake of turning this into a party election," said Democrat Goldman. "That's never been the way to do it in Virginia."

Besides the ticket, Republican losers today included Dalton, who cannot succeed himself, and former governor Mills E. Godwin, whose lukewarm endorsement of Coleman did little to offset large-scale defections by the conservatives he had once claimed to lead.

The lukewarm feelings were shared even by party regulars. "We had a hard time lighting fires under state party people," said one Republican, who recalled a September meeting at which Coleman strategists were told "no one is willing to bleed yet for Marshall Coleman."

The seeds of discontent were sown long before this year's campaign. Coleman had alienated many party regulars in 1977 by upsetting the favored Wyatt B. Durrette of Fairfax County for the GOP's state attorney general nomination. Coleman continued to anger old-line conservatives when he defeated Democrat Edward Lane that year by resurrecting Lane's segregationist past, a past many of the old guard had shared.

Durrette found his hopes for attorney general buried again this year by the Democratic sweep. While he finished far above his two running mates, Durrette still fell 27,000 votes short of victory, losing 49 to 51 percent to Democrat Gerald L. Baliles of Richmond.

There were also widespread complaints about Coleman's media campaign, which party regulars criticized as too negative, and field organization. All of that was dismissed by Coleman supporters as second-guessing. "If we had won, the party people not in love with Marshall would have been standing in line for their inaugural tickets," said Matt Wirgau, who headed Coleman's field operation.

The size of Robb's victory came as a shock to Republicans who had been buoyed by a weekend survey by pollster Richard Wirthlin putting Coleman ahead by four points. Wirthlin could not be reached for comment today, but Robb pollster Peter Hart, who had showed the candidates virtually neck-and-neck over the weekend, said it was Robb's ability to project a trustworthy and responsible image that made the difference.

"Robb came across as strong and solid," said Hart, while Coleman "didn't come across as a solid human being."

Other Democrats suggested Coleman never appeared comfortable as a hard right candidate. Said William Wiley, a Democratic strategist: "Marshall tried to be something that he wasn't, an ultra-right-winger."