A honey-voiced grandmother, accompanied by a half dozen Secret Service agents, came to this rainy Virginia town today, some 1,500 miles from her home, in an effort to help her prosperous son-in-law get a part-time job in Richmond.

Lady Bird Johnson, whose warm Southern style made her seem as much a part of Culpeper as jonquils are a part of spring, may turn out to be son-in-law Charles S. Robb's secret weapon in his bid for the Democratic nomination for lieutenant governor if her reception here today is any indication.

Robb is running against an Arlington lawyer, Del. Ira M. Lechner, and Richmond Del. Richard S. Reynolds. Neither has announced plans for their mothers-in-law to campaign.

Her visit to Culpeper with Daughter Lynda Robb - the candidate spent the day campaigning elsewhere - was part of a series of visits Mrs. Johnson said she will make on Robb's behalf. In between the public appearances she will make before the June 14 primary. Mrs. Johnson said she will stay at the Robbs' Mclean home to take care of her two grandchildren there, to "provide continuity for the children when Lynda needs to travel.

Campaign aides said Mrs. Johnson, whom Lynda described as "the best campaigner in the family," specifically asked to visit Culpeper. It was here that Lyndon Baines Johnson began his 1960 whistlestop campaign train journey through Southern states, and it was here that he uttered the famous line: "What has Richard Nixon ever done for Culpeper?" which his daughter alluded to in her brief speech today.

Reminders were much in evidence today that LBJ, in 1964, was the only Democratic presidential candidate to carry conservative Virginia since 1948. Several local residents who attended a "Dutch treat" lunch for Mrs. Johnson and her daughter brought out yellowing photographs of themselves with one or other of the Johnsons. Conversation overhead during the lunch and compliments tendered to Mrs. Johnson in the reception line afterward contained frequent references to LBJ being "the best President we ever had." But one man was heard insisting to a friend that actually Harry S. Truman was clearly the greatest.

Two young women, sisters who as 4- and 6-year-old children had presented a bouquet to Mrs. Johnson during the brief 1960 stop, re-enacted the scene at the train depot today, although the chill rain hurried things along a bit. One of the young women said she was now attending a local community college; her sister said she had graduated from the University of Virginia and hoped to attend law school and go into politics.

About 140 people paid $4 each to attend the lunch, including a 62-yer-old black man who was introduced and said he had worked as the Johnsons' butler in the early 1940s.

"I've been thinking about this for a whole week," said John Glascoe. "As each day approached I got more and more nervous. This morning instead of taking a little time off from work I decided to take off the whole morning. An opportunity like this doesn't come but once in a lifetime."

After leaving the train depot, Lynda Bird and Lady Bird visited a 200-year-old church that is being restored. "I just love old churches and museums like this," Mrs. Robb said as she was shown around the building. They were told how the Yankees had destroyed almost everything during the Civil War, although one had sent $100 later to help pay for the repairs.

The two had their pictures taken for the Virginia Churchman, a publication that goes to about 50,000 families in the Episcopal diocese.

At an early afternoon press conference, Mrs. Johnson adroitly sidestepped a question on her opinion of President Carter's first months in office. "Since Lyndon left office I really haven't talked about politics," she said. "I'm certainly no authority . . . Gosh knows I wish him all the luck in the world."

She also said she didn't know anything about Virginia politics, but "when it comes to your daughter and son-in-law you simply want to do what you can."