Virginia Lt. Gov. Charles S. Robb was nominated today as the Democratic candidate for governor and immediately promised to include disappointed liberals in "a serious, issue-oriented campaign" to recapture the office that Democrats last won 16 years ago.

The unanimous selection of Robb, 41, whose star quality comes from being the son-in-law of former President Lyndon B. Johnson, combined with the nominations of former Portsmouth Mayor Richard J. Davis for lieutenant governor and Richmond lawyer Gerald L. Baliles for attorney general, gives the party a ticket with solid conservative credentials. They will face a Republican slate that will be picked here next weekend and is certain to be headed by state Attorney General J. Marshall Coleman.

Still at issue for the Democrats is how many supporters of Arlington liberal Ira Lechner, whose bid for lieutenant governor failed by a 3-to-2 margin, will sit out the election rather than work for a team some of them believe will be a mirror-image of the Republicans.

Lechner personally endorsed the ticket today and said he would accept a new leadership position within the party as vice chairman, but added he did not know how many of his followers he could persuade to unite behind the nominees. Robb and his strategists have conceded they need those followers to help to beat Coleman and the GOP.

But at least for now, Robb and his followers have accomplished what they came to this resort city to do -- conclude their convention with a minimum of rancor among the warring ideological factions that in the past have divided the party and contributed to a decade-long skein of electoral defeats.

"You came into a convention with a tremendous reservoir of frustrations, loose ends and raw nerves that had to be placed in perspective," said Robb, as he watched with wife Lynda and his three daughters from an office above the convention hall after his nomination.

"We can't hope to ever satisfy everyone because there are deep differences in style and issues among many of the people here," added Robb. "But I think we're coming out of here as genuinely unified as we could."

The ticket does offer the geographic spread that Democrats believe they have to have for victory in the November elections. Robb, a McLean lawyer, should help the Democrats gain votes in the Washington suburbs, Davis, in the big Tidewater region, and Baliles in the Richmond suburbs, one of the most conservative sections of the state.

Robb has been a target for the party's progressive wing ever since he catapulted his way to the lieutenant governor's office in 1977 using his personal wealth and the public fame of the LBJ convention. But as the only Democratic statewide winner that year, Robb has been the party's gubernatorial heir apparent for four years, a fact many party liberals accepted reluctantly.

Their willingness to accept the conservative Robb in hopes of retaking the Governor's Mansion was a key element in the relative peace that reigned at this convention, party strategists said. The other element was Robb's own willingness to give the dissidents a voice in the party despite advice from some regulars who wanted the liberals turned away with nothing.

"Chuck handled this thing very, very skillfully," said Lechner supporter Paul Goldman. "He had a role to play and he played it well. At the same time, we felt we simply had to bury once and for all the image of liberals as party wreckers."

But even the agreement to give Lechner a new party position did not end all the bitterness that lingered between Lechner and the Davis supporters, many of whom had opposed the peace-making offer.

"For three weeks Davis has known he would win here," said Lechner. "Yet there wasn't one phone call, not one effort to reach out to me. They just wanted to grind me into the dust."

From Davis' view, it was a matter of Lechner failing to approach him. "In my whole life," Davis said, "I have never talked to him, privately and personally. He came to only four of the 374 meetings I conducted" as state chairman.

Some liberals said, despite the last-minute show of unity, the convention will not help Robb reunite the party. "Liberal leaders will be for him but whether we can motivate others is another matter," said George Rawlings of Lorton, chairman of Northern Virginia's 8th Congressional District delegation. "This convention did little to help and many of the new delegates were appalled" at the treatment given to challenges posed by Lechner, Rawlings said.

But others, citing the need to break the Democrats' long dry spell in Virginia as well as to send a hopeful message to party members nationally, said they were satisfied with the resuls here. "With Reagan in the White House, we desperate need a victory and I'm willing to give up a lot if that's what it takes," said liberal State Sen. Clive DuVal of McLean.

"There aree a lot less lemmings in this convention," said former Virginia attorney general Andrew P. Miller of Alexandria, who has often blamed his narrow defeat for U.S. Senator in 1978 on lack of help from party liberals. "I've been at party meetings where the first thing some people asked was 'Where's the cliff?' but not this time."

Having launched his nomination in an atmosphere of unity, Robb must now turn his attention to opponent Coleman, 38, the aggressive, self-confident attorney general whose handsome features and conservative views are a close match to Robb's.

In his acceptance speech, personally filmed by media advisor Robert Squirer for use in a future television advertising, Robb contrasted his own solid style with Coleman's.

"Virginians don't need more empty rhetoric and slick phrases. They don't need glib jokes, they need new, thoughtful ideas," he said.

Robb also warned about "those who seek to divide our people through hatred and scare tactics," but did not identify whom he was referring to either in the speech or in a later interview.

Robb's nomination and speech, timed for maximum exposure in the state's major television markets, kicked off today's session, after which the two contested races were decided. Voting for attorney general was the closer of the two with Baliles beating Del. Erwin S. (Shad) Solomon of Bath County in Western Virginia by 1,961 to 1,599. Davis outpolled Lechner 2,074 to 1,258, and a third nominee, State Sen. Dudley J. (Buzz) Emick Jr. of Botetourt had 237.

By mid afternoon both contests had ended amicably. Many Democrats said they expected the relatively peaceful session would contrast with next week's GOP convention, where a showdown is expected between New Right and fundamentalist forces and Republican regulars over the party's lieutenant governor's nomination.

"Wait till the born-again crowd gets here," said Arlington Democrat John Milliken, who added the Republicans could be the ones practicing what he called "the old Democratic political syndrome" of feuding.