Virginia Lt. Gov. Charles S. Robb comes to the Democratic Party's state convention here this weekend with the gubernational nomination firmly in his grasp but still lacking the unity he needs to win in November.

A feverish week of closed-door meetings and negotiations designed to forestall a liberal challenge to the party's leadership have left Robb and his backers troubled about what the party's liberalw ing will do during the two-day session.

They fear that Ira Lechner, a combative, liberal Arlingtonian whose bid for the party's nomination for lieutenant governor seems certain to fail, will turn the convention into a liberal-versus conservative brawl reminiscent of the past party slugfests that have helped turn Virginia's Democrats into an endangered political species.

Lechner, a former state legislator who has attracted many supporters of populist Henry E. Howell, has turned his attention to seizing the state party's chairmanship by ousting Robb's choice, Virginia Beach Del. Owen Pickett, a conservative.

At a secret session last weekend at his McLean home, Robb offered to create a new post of party vice chairman for Lechner, an offer repeated yesterday by Robb campaign chairman David Doak. So far, Lechnerhas turned them down.

Nonetheless, Robb forces still hopes to strike a deal with Lechner before a showdown over the chairmanship Friday night at the convention's opening session.

"We know Ira can benefit our campaign," said Robb press secretary George Stoddart. "Chuck seeks to unite both sides of the party. For too many years, those who are ideologically strident have lost elections. We're sick of losing."

To break a 12-year Democratic losing streak in gubernatorial elections, Robb must defeat Republican Attorney General J. Marshall Coleman, an aggressive, media-oriented candidate who offers much of the same combination of conservative programs and personal star appeal as does Robb. Coleman will be nominated on this same convention floor next weekend when the Republicans come to town.

Many Virginia politicans say that Robb emerged surprisingly strong from the preliminary campaigning that began in March when the two men formally announced their candidacies. To continue that momentum, Robb hopes to present a picture of Democratic harmony this weekend that will sharply contrast with the GOP convention, where born-again New Righters are set to battle Republican regulars for the party's lieutenant governor's nomination.

Robb has made some progress in uniting such disparate elements as blacks, labor union members and party conservatives behind his own candidacy. That success is symbolized by the fact that among those seconding his nomination will be Richmond Sen. L. Douglas Wilder, Virginia's highest-ranking black elected official, and Arlington Del. Warren Stambaugh, a Lechner supporter and legislative maverick.

Still, Robb's supporters concede he needs a cooperative Lechner to make the party whole again. "It's important for the party to send a clear signal to the people supporting Ira that they are needed in the fall and beyond then," says Arlington Board member John Milliken, who is supporting both Robb and Lechner and who helped arrange last Saturday morning's meeting.

Lechner said he came away from the two-hour session with Robb believing that "Chuck and his supporters are genuinely concerned. He wants the support of blacks, labor and progressives in Northern Virginia. The question is, what price is he willing to pay for that. The conservatives say [pay] nothing. If he's even seen having dinner with me, the shock waves roll at the way from Main Street [Richmond] across the Southside."

While Lechner continues to negotiate with Robb, he also is talking to the two rivals for the party's nomination as attorney general, both of whom are still looking for enough votes to put them over the top. Lechner had met with both front runner Gerald L. Baliles of Richmond and opponent Erwin (Shad) Solomon of Bath County, but said "neither has agreed to support me for chairman."

"But I've got something they both want -- 500 delegates uncommitted in that race," Lechner said.

Lechner got those 500 delegates, and 600 others, in a surprisingly strong attempt to best former Portsmouth Mayor Richard Davis for the second spot on the ticket. Lechner says he still plans to challenge the credentials of nearly 500 of Davis's 1,700 delegates at the Friday night session.

The Davis camp says it is confident of defeating that challenge but charges and countercharges of wrongdoing by both candidates in the delegate selection process have left had blood between them.

Since Mr. Lechner has said he's still actively pursuing it [the nomination] and making false charges, it doesn't seem like there's any room for compromise," says Davis campaign manager Robert Watson. "The problem here is personal ego. Ira's supporters may be satisfied when it's over but Ira wants something for himself and I don't see him walking away with anything."

After a strong week of campaigning last March following his formal announcement, Robb lowered his profile for more than a month, while Coleman conducted a blitz of press conferences, position papers and $250,000 worth of television and radio ads. Robb emerged two weeks ago with a 50-page state economic plan that won points for its seriousness, then took on Coleman in a debate that most observers believe Robb won.

Still, despite new confidence, few Democrats say they are certain of victory in November.

"We've got to recognize we face some strong Republican opposition," says Darrel Martin, Baliles' campaign manager. "Democrats can win in the fall but only as one party, not as three."