The moment that Joe Alder, a barrel-chested, slow-talking lawman, strode from his gray patrol car outside the Lunenburg County Courthouse and introduced himself to the Democratic candidate for lieutenant governor, a star was born.

Political consultant Paul Goldman had picked the pre-Civil War courthouse here in the heart of Virginia's conservative Southside as the background for a television commercial designed to combat Republican contentions that Democrat L. Douglas Wilder of Richmond was soft on crime.

And when Goldman observed the chance meeting between the small-town southern police officer and the urban black politician, Goldman asked the deputy sheriffs he had gathered for the occasion to step aside, and threw away the prepared script. In Alder, Goldman had found his man.

Wilder, whose victory Nov. 5 catapulted him into the national spotlight (he was interviewed this week by People Magazine) said the 30-second TV spot featuring Alder did "nothing less than win the election for me."

Back in Kenbridge (population 1,200) where the 32-year-old Alder is one of four police officers, he said he has "caught a little static from the people who didn't vote for Doug."

For the most part, he said his TV role has brought him a lot of good-natured teasing and admiration, especially from black residents who were "tickled to death" by his support of Wilder, the first black to win a major state office in the South.

When Alder's 2-year-old son Michael saw the ads, he would point to the TV and shout: "Daddy, Daddy."

His wife Deborah, a nurse, "thought it was funny. She couldn't get used to seeing me on TV," the officer said.

Alder said he was proud to say he voted for Wilder, although that doesn't necessarily represent the consensus of the Kenbridge force. Sgt. Frank Whitlow, Alder's frequent patrol partner, said "no comment" when asked what he thought of Alder's TV role.

Police Chief Jesse C. Carter, who said he wasn't consulted in advance about Alder's performance, said he figures "a little publicity can't hurt" the town, about 30 miles from the North Carolina border.

Alder was given four lines to recite, lines that Goldman scrawled on a yellow legal pad as he leaned on the hood of the officer's cruiser. But with no acting experience, and working under a 95-degree sun, Alder took more than an hour to complete the spot.

"We were doing it one line at a time," said Alder, who wasn't paid for his role. "The campaign manager was playing it by ear."

Having memorized the script, Alder won't soon forget it. Between sips of coffee at his favorite restaurant, he recites: "I'm a working policeman. I put my life on the line every day. That's why we need people in public office we can trust. The Fraternal Order of Police endorses Doug Wilder for lieutenant governor."

The effectiveness of Alder's performance, noted by syndicated columnists Mark Shields and Carl Rowan, as well as many Virginia politicans, was the officer's believability. The ads played so frequently on TV that when Republicans attacked Wilder on crime, "it wasn't Wilder they were attacking, it was the TV ad," said Goldman.

The 32-year-old officer isn't a member of the Virginia Fraternal Order of Police. But he pronounced "loo-tenant" governor with a down-home authenticity, and said his lines with sincerity.

"I knew we weren't supposed to mix with politics," Alder said, "but I asked Jim Edmunds, who is the town attorney, and he said it was okay."

Edmunds, a former Virginia state senator, was hardly a disinterested party. He is the Democratic chairman of the 5th Congressional District.

Wilder said the beauty of Alder's ad was that "it wasn't set up -- and he came over that way -- not as a slick politico.

"It established an issue for me the Republicans usually have going their way, strong on law and order," said Wilder.

"Alder was the image of a guy who was not supposed to be supportive of my candidacy. He destroyed the myth that the Joe Alders wouldn't support me. He was mainstream, blue-collar and rural," the lieutenant governor-elect said.

Alder's endorsement was so good that Goldman used a portion of it on another Wilder commercial, featuring Gov. Charles S. Robb. In all, Alder appeared in about half of the $480,000 worth of TV time purchased by Wilder.

Even with Alder's support, Wilder carried Lunenburg County by only three votes, 1,542 to 1,539. But the lieutenant governor-elect said, "Even that is not supposed to happen in Southside."

Alder had no idea he was going to wind up in a political TV ad when he went to the courthouse that day. He said that he believes he got the speaking part instead of the deputies (who were shown shaking hands and walking with Wilder) was "because their uniforms were brown and mine was blue."

Alder said Wilder was campaigning for the legislature while Alder was serving as a teen-age page in the House of Delegates in 1968. "He was shaking hands and politicking then," said Alder.

The officer said he recalled that Wilder had received the endorsement of the FOP. The spots gave the impression that he was a member of the group. "Mr. Edmunds said it was okay to say that," Alder said.

Alder said he was told he might get an invitation to the inauguration ceremonies Jan. 11 in Richmond, and Goldman said that "he's going to be an honored guest."

"I'd love to go if I can," said Alder, who like many veterans of the long poltical campaign, took a few days off after the election. "I went bear hunting right after I voted, and didn't get back until Sunday night."