One year after he was credited with resurrecting the Virginia Democratic Party, Gov. Charles S. Robb suffered his first major setback last week in Richard J. Davis' defeat for the U.S. Senate.

The 43-year-old governor, whom supporters are fond of touting as a prospective vice presidential candidate in 1984 or beyond, had thrown himself enthusiastically into the Davis campaign during its final month, giving fund-raisers, taping television commercials and criss-crossing the state to deliver stinging attacks on the ultimate victor, Republican Paul S. Trible Jr.

That flurry followed a series of campaign forays around the country in which Robb, the son-in-law of President Johnson, raised money and campaigned for Democratic candidates in Nebraska, Iowa, New Mexico and a half-dozen other states. Had he managed to help pull off a come-from-behind political triumph for Davis, his lieutenant governor, Robb's stock in Democratic circles would have soared, many state politicians said.

"Oh my God, his salesmen would have been beating the drums making him out to be an unbelievable world-beater," said State Sen. Dudley J. Emick, (D-Botetourt). "Now, Davis' loss is going to quiet those tom-tom drums for a while."

"He went to bat for Davis and he didn't win," added Del. Vincent F. Callahan Jr. of Fairfax, the Republican leader of the House of Delegates. "Some of the luster is off. It takes away from his image of invincibility."

Robb's confidants were well prepared to dispute that assessment when the dust had cleared late last week. Robb had been equally active in the state's congressional races, where Democrats picked up three Republican-held seats. And in the House of Delegates, where the GOP had once hoped to pick up as many as a half-dozen seats due to reapportionment, Republicans were held to a stand-off, giving Robb the same 66-to-33 majority to work with when the General Assembly convenes in January.

"Look at it this way," said George Stoddart, Robb's press secretary. "A lot of people were talking about how the national election was a referendum on Ronald Reagan and the Republicans lost 26 seats in the House. If Virginia was to the same extent a referendum on Robb, we didn't lose any seats in the state legislature."

Robb didn't lose any time last week putting a bit of distance between himself and the candidate for whom he had worked so hard. In a news conference the day after the election, Robb suggested that he had always had "reservations" about his party's nominee because, in contrast to the hard-charging Trible, "Dick Davis didn't have that fire in the belly."

And while repeating his by-now-ritualistic denials of national political ambitions in 1984, Robb emphasized that he had no plans to abandon the cross-country political journeys that, skeptics argue, are principally designed to position him in case lightning should strike and he is picked as vice presidential nominee.

"I'm going to continue to pursue my responsibilitites to my party locally and nationally," he said.

The upshot of Tuesday's election is to subtly alter the state's political equation for the next few years, restoring Virginia as one of the strongly competititive two-party states. Robb, while slightly tarnished, remains by most accounts the most formidable, and popular, political figure on the scene. One election day exit poll by the Associated Press and WRC-TV found that 65 percent of the Virginia electorate believe Robb is doing a "good" or "excellent" job as governor compared to 47 percent who thought the same about President Reagan.

Yet Republicans, who a year ago were demoralized by the Democratic sweep of statewide offices led by Robb, are now resurgent and poised for combat. The word quickly began circulating in party circles last week that former Gov. John N. Dalton, now a corporate lawyer in Richmond, is laying the groundwork for another campaign for the governor's mansion in 1985.

It is a race in which Dalton has been instantly rated a strong contender, if not a favorite, against any of the leading Democratic hopefuls, such as Attorney General Gerald L. Baliles and Del. Richard M. Bagley (Hampton), the chairman of the House Appropriations Committee.

More immediately, there is 1984, when Republican Sen. John Warner will be up for reelection. Alexandria lawyer-lobbyist William G. Thomas, who is Robb's closest political adviser, had begun planting the seeds for a race against Warner earlier this year, a race in which he has undoubtably hoped to benefit from his close association with the popular governor.

Yesterday, with Robb's ability to transfer that popularity newly in doubt, Thomas sounded still interested, if somewhat cautious. "I will between now and then 1984 be looking at it . . . to see whether it's a viable thing to do," said Thomas. "I want to see the strengths and weaknesses of John Warner and get a better feel for the political climate."