Consevative Democratic supporters of Virginia Lt. Gov. Charles S. Robb tonight beat back a bitterly contested drive by dissident liberals to capture a major leadership position for themselves at the party's first gubernatorial nominating convention in 80 years.

The defeat clears the way for the party's conservative and moderate wing to nominate a state ticket tomorrow of Robb, former Portsmouth Mayor Richard J. Davis for lieutenant governor and Richmond Del. Gerald Baliles for attorney general. But it means the exclusion from power of party liberals, many of them from Northern Virginia, whom Rob concedes the ticket must attract in order to win in November.

The liberals' leader, former Arlington legislator Ira Lechner, who had hoped to take the party's second slot from Davis, conceded tonight he had no chance of winning. The convention rejected his challenge to the credentials of move than 700 Davis delegates.

Lechner also abandoned an attempt to oust Robb's handpicked choice for the state party chairman, conservative Virginia Beach Del. Owen Pickett. When it came time for the delegates to vote on the chairmanship tonight, Pickett was reelected almost unanimously without any mention of Lechner's veiled 11th-hour bid.

As for Lechner's fast fading hopes to win the lieutenant governor nomination, Lechner charged that the Davis delegates had been improperly selected. "They're whitewashing what happened and ignoring the party rules," he said. "It's obvious they're trying to have a deck so stacked we have no chance."

After a series of closed-door meetings tonight, Lechner agreed to accept Robb's offer of a party vice chairmanship that would give him authority over voter registration and put him in charge of dealing with traditional liberal interest groups that he represented here.

Although Lechner accepted the party post, he had warned earlier that other members of his coalition of labor union members, teachers, blacks, and other liberals might not follow his lead.

"I can't promise my people will do the same because I don't control them," said Lechner.

Robb, seeking to become the first Democrat elected to the governorship since 1965, still appeared confident today that he could unite the party's warring factions. But despite his efforts, the antagonism evident on the convention floor today threatened to spoil the unity show that Robb and his supporters hoped to present here.

"When they [the conservatives] get what they want it's called unity but when they don't call it party-wrecking," said Lechner supporter Paul Goldman. "It's an incredible double standard."

Some delegates circulated a letter Lechner wrote two weeks ago accusing Davis' supporters of "rampant fraud . . . fear, intimidation, and Watergate-style dirty tricks." Those charges caused an angry reaction from Davis' people.

"You're seeing a very sad case of someone putting personal interest above party interest," said Davis campaing manager Robert Watson of Lechner. "Those responsible for making those charges shouldn't even be on this floor."

Robb's strategists have conceded all along that they need the liberals, who have been among the party's most loyal foot soldiers and campaign workers in recent years, in order to break the GOP's long winning streak and defeat Robb's certain Republican opponent, Attorney General J. Marshall Coleman.

To underscore that need, Robb effusively praised Lechner today and promised him and unspecified role in the party. He added that he expected the convention to display "the diversity of our party in all its splendor."

But that diversity quickly turned to open antagonism today as the Lechner and Davis forces traded charges. Some Northern Virginia party regulars claimed Lechner supporters had purged them from the party's state central committee.

"Ira did it to me," said former Fairfax party chairman Emilie Miller, one of three area leaders ousted two weeks ago even though she claimed to be neutral in the lieutenant governor's contest. "He's not just trying to get the nomination, he wants to run the party too."

Lechner supporters denied the charge. "Nobody's purged anybody," said George Rawlings, a Northern Virginia liberal who lost his seat as a Democratic national committeeman to party conservatives last year but is 8th District convention chairman here. "They did it to us last year. Now it's our turn to win some back."

Davis's supporters claim they have more than 1,800 delegates, enough to assure a first-ballot victory in a convention likely to attract about 3,500 delegates. The Lechner forces claim between 1,300 and 1,400 and had hoped to make added inroads tonight through credentials challenges based on what they claimed were irregularities at nearly 50 of the party's 157 mass meetings last month.

But in rapid-fire succession, the convention voted down the challenges either by voice vote or 2-to-1 margin.

Arlington Del. Warren Stambaugh, who had battled unsuccessfully in Lechner's behalf before a three-hour credentials committee meeting earlier in the evening, told the convention, "If we don't follow the rules, we might as well tear them up," whereupon he ripped to shreds of piece of paper to the cheers of the Lechner delegates.

But credentials committee chairman Glenn Croshaw, a law partner of party chairman Pickett, said "the proceeding were fair," even while conceding that the results were based on political loyalties rather than the merits of the Lechner complaints.

While realizing they would lose the credentials challenges, the Lechner forces has hoped to convince some of Davis' black and labor union supporters to join a move to oust Pickett, a conservative with few ties to the party's liberal wing. But Lechner conceded tonight he had been unable to budge those delegates and abandoned the attempt.