Gov. Charles S. Robb and other Virginia Democrats yesterday attributed Richard J. Davis' defeat for the U.S. Senate in large part to Davis' reluctant and occasionally indifferent campaign style, saying he failed to convince voters that he really wanted to succeed Harry F. Byrd Jr.

Assessing their party's loss to Republican Rep. Paul S. Trible, Robb and and others said that they were not criticizing Davis' abilities or qualifications for office. But they emphasized that the 61-year-old Davis, who had first spurned the party's nomination when it was offered him last spring, suffered in comparison with the aggressiveness, intensity and political ambition of the 35-year-old Trible.

"There is what is sometimes referred to as fire in the belly," Robb told a news conference. "It is clear that Paul wanted very much for a long, long period to be a United States senator . . . . Dick Davis didn't have that fire in the belly and that was one of the reasons that I had some reservations about him as a candidate -- never as a United States senator."

Davis' attitude was one of many factors that were cited yesterday in explaining Trible's triumph. Many mentioned the huge Republican Party's head start in organization and fund-raising, a lead that ultimately allowed Trible to outspend Davis by more than $700,000 and finance last-minute direct mail and media blitzes that overwhelmed the Democrats. Others cited the essential conservatism of Virginia voters, an electorate that, according to exit polls, still looks kindly on President Ronald Reagan and is willing to "stay the course" on the economy.

Numerous party leaders focused their crticism on Davis' own failings as a candidate. A millionaire mortgage banker, who would take one day each week from the campaign to attend to his business affairs, Davis never disguised the fact that he was more interested in serving as governor than moving to Washington as a senator. In one newspaper interview, Davis refused to rule out the possibility that, even if elected to the Senate, he might run for Robb's job in 1985 rather than serve out his six-year term.

"If I were not to win, it would not be the end of the world for me," Davis occasionally said on the campaign trail. "It would not be the end of the world for Virginia."

"The question created for many party people was how seriously did Dick want it," said state Sen. Richard Saslaw (D-Fairfax). "When you run for another office, you've got to eat, sleep, breathe and drink it. It has to be totally consuming. I'm not sure it was for Dick."

Another Fairfax Democrat, state Sen. Adelard L. Brault, echoed much the same theme. Davis' easygoing style and flip campaign statements "gave people the impression that maybe he just doesn't care . . . . Trible is young and exuberant or tries to be. Sometimes he's artificial, but that contrast might have hurt Dick Davis."

Davis paid an early-morning visit to his campaign headquarters and then sequestered himself in his Richmond apartment for the rest of the day.

As Republicans saw it, Davis' indifference was reflected in a campaign strategy, including a heavy dose of negative radio advertising, that attempted to drag down Trible without ever giving the public a positive reason to vote for Davis.

Davis rejected numerous opportunities for face-to-face debates and, other than general statements over high unemployment and record budget deficits, he avoided specifics on most issues. "They the Davis campaign failed in the final days when they were giving people good reasons to vote against Paul, but they weren't giving people a reason to go with Davis," said Wyatt B. Durrette of Vienna, last year's GOP candidate for state attorney general and a Trible adviser.

Indeed, this was even reflected in some of Davis' "positive" media. Democratic media consultant Robert Squier noted today that the campaign invested heavily over the final weekend in a TV spot that consisted almost entirely of Robb speaking on behalf of Davis -- without any words from Davis himself. The idea was to capitalize on the political strength of Robb, who polls show is by far the most popular political figure in the state. But even Squier acknowledged today that the impact of such spots is limited. "We were maybe a little heavy with him Robb ," Squier said.

Most Democrats, though, pointed to tens of thousands of direct-mail letters attacking Davis by New Right groups, such as the National Rifle Association and the Rev. Jerry Falwell, the television evangelist from Lynchburg. There was also speculation about the impact of the repeatedly shrill attacks on Davis from the staunchly conservative Richmond newspapers as well as a rumor that swept throught the Richmond area over the weekend that Robb was ready to appoint state Sen. L. Douglas Wilder, the state's senior black official and a liberal, lieutenant governor if Davis won the Senate seat.

Robb himself said he encountered the rumor repeatedly in the last few days and said he believed it had an impact. "Everybody was talking about it," the governor said. "It was a somewhat insidious tactic that I deplore."

In the end, the race was a tight one. Final tallies today showed Trible had 723,875 votes or 51 percent to Davis' 689,502 votes or 49 percent. The crucial figures came in Northern Virginia, which Davis only carried by a 52-to-48-percent margin, when his strategists were counting on 55 percent.

Trible, meanwhile, swamped Davis in his own 1st Congressional District, a normally Democratic area consisting of Newport News and Tidewater Virginia, which he carried with 59 percent of the vote. Trible's 28,000-vote margin in the 1st, a district that Robb carried by 15,000 during last year's governor's race, accounted for more than three-fourths of his statewide plurality.

Democrats at least took some consolation in Trible's slim statewide margin, a tightness that Robb described as remarkable given the disarray in which the party found itself last spring when it was literally unable to find any candidate for the Senate. "Five and a half months ago, it was vitually impossible for any Democrat to have even a chance," said Robb. "The Republican National Committee thought this was going to be a piece of cake."

In the final analysis, though, it was a personal triumph for Trible -- an aggressive, hard-charging politician who first began campaigning for the Senate seat two years ago, before Byrd had announced his retirement. Today, only hours after his victory, he was at it again, showing up at 5:30 a.m. at the Newport News Shipbuilding and Dry Dock Co. to thank workers on the early shift.