He still looks a little like the boy on those old family post cards they sell near the LBJ Ranch, and it wasn't so long ago that he was just Lyndon's son-in-law.

But last night, Virginia's Gov.-elect Charles S. Robb had his own coming out party here before an audience of enthusiastic Democrats who hope to do in Texas what Robb did in Virginia.

They might have called it "The Chuck and Lynda Show," and even discounting for a home-town audience, it was a boffo performance, full of one-liners, loving compliments, political advice, and a marvelous ad that never got on television during the campaign. An exuberant Lynda Robb captured the evening's mood best when she passed on a family homily, "Lose your breath, lose your turn," and for the better part of an hour, as one gulped, the other jumped into the breech.

With Lady Bird Johnson and Lynda's sister Luci Nugent in the audience, Robb praised his "loving and supportive family" for their help in his campaign and then paid the Johnsons the ultimate compliment. "I've told people there's simply no reason a fella can't marry well," he said. "I did."

Lynda, whom Robb introduced as "a lady who will make a dynamite first lady," told the crowd that her sister had first campaigned in Virginia wearing a cast and was so convincing, "We got the handicapped vote."

The compliments were flying so fast that even Robb had to duck occasionally. One friend called him "the most attractive young Democrat in the nation today," and Rep. J.J. (Jake) Pickle, who has been Austin's congressman for 19 years, rose to describe Robb's victory as "one of the most hopeful signs Democrats have had nationally in the last 10 years."

"I hope you know what a gem you've got in Jake Pickle," Robb quickly responded.

The Robbs appeared totally at ease throughout the reception, which began in a bank lobby and ended in a bank auditorium. They posed for pictures -- "If you can get her to pose, you're doing well," Robb said to a photographer focusing on his wife -- and greeted old friends and tried to get their lines straight before the performance began.

"Now who was it who came to Texas from Virginia besides Sam Houston?" Robb asked his wife as they headed upstairs in the elevator.

"Stephen F. Austin, as in Austin," Lynda Robb replied. "But that's my line."

By the time the show began, her husband had stolen it, though not adroitly.

They sat behind a table in the front of the auditorium and held hands while the audience asked questions, and occasionally Lynda would pull on Robb's arm to get him to stop rambling, or would amend one of his answers. Asked if he thought the Virginia election was a referendum on President Reagan, Robb said he did not. Lynda quickly took the microphone out of his hands. "That's not to say our opponent didn't try to make it a referendum on Reagan," she said. "He tried constantly."

She jokingly called her husband's long position papers "marvelous if you have insomnia," proudly noted that he was such a fiscal conservative that they had ended the campaign almost in the black and bragged about their secret weapon for counteracting the likes of Reagan, Vice President Bush and other Republican heavies who had campaigned in behalf of GOP candidate J. Marshall Coleman.

"Whenever they brought in any of those sparklies," Lynda said, "we brought in our daughter Jennifer."

The Robbs also brought to Texas another weapon they never had to use, a television commercial designed to answer charges that Robb's campaign was relying on Texas friends and Texas money. It showed Robb's opponent at the Petroleum Club in Houston raising money, and ended with a smiling Coleman shaking hands with someone and saying, "Thanks a million." The audience, which included former Sen. Ralph Yarborough and President Johnson's press secretary George Christian, roared with laughter.

Robb told his Texas audience he wouldn't presume to advise them on how to win the governor's office back from Republican Bill Clements, but urged them to try to make the Democratic party "the party of the people, the party of inclusion."

"We went out and tried to recruit new folks or a few old folks . . . who had strayed from the way like lost sheep," he said. "We cast our net broadly, and we convinced people that on the basis of what this party stood for there was a role for everyone who wanted to subscribe to the principles of the party."

At the end, the Robbs, who said they already have sent out 20,000 thank-you notes to supporters, invited everyone to "the holy city of Richmond" for inauguration day on Jan. 16.

And when it was over, no one in this polite Texas audience had asked the new governor whom he had voted for in the 1964 presidential election -- a question Robb dodged throughout his campaign.