Virginia's conservative old guard rallied today in an effort to persuade Sen. Harry F. Byrd Jr. to reconsider his decision to retire from Congress next year.

Former governor Mills E. Godwin and two other leading conservatives began a petition drive to place the independent senator's name on the November ballot, saying the chances were "better than 50-50" that he would change his mind about not running for reelection.

Godwin said today that he spoke to Byrd on the telephone and that "while he has made no commitment, it is my thought that, if the people of Virginia make known their wish that he remain in the Senate, the chances are better than 50-50 that he may reconsider his decision."

Conservatives used similiar tactics in 1973 to persuade Godwin, a former Democratic governor, to come out of retirement and run as a Republican against a liberal Democrat, Henry E. Howell of Norfolk. Godwin, with support of the state's business community, narrowly won a second term as governor.

The draft-Byrd movement, which is being organized by Godwin, former Democratic representative Watkins Abbitt and former Democratic delegate W. Roy Smith, could drastically alter the prospects for Rep. Paul S. Trible, the apparent Republican Senate nominee who has been eagerly courting Byrd's supporters.

Some Democrats, whose party has been torn apart in the confusion over who will seek Byrd's seat, were stunned by the draft-Byrd effort.

Gov. Charles S. Robb, who has been criticized for his party's disarray, used the draft-Byrd statement to take a shot at the Republicans. It is "hardly a ringing endorsement of the apparent Republican nominee," Robb said. "It could go a long way towards explaining why their sole contender has had so much difficulty raising money even though he has been seeking the Senate seat for almost two years."

The conservatives immediately began circulating petitions throughout the state in an effort to collect the 10,756 signatures needed by June 8 to place Byrd on the ballot as an independent. While Godwin said the task would be "almost a Herculean job," he expected that Virginians would respond in droves.

"It would be the best thing that could happen to Virginia to keep him in the Senate," said Godwin, who is regarded as one of the senator's closest advisers.

The effort to draft Byrd threatens to dramatically transform a Senate campaign that has dominated Virginia politics since Nov. 30 when the senator announced he would not seek a fourth term. He has been in the Senate since Nov. 12, 1965, when he was appointed to succeed his late father, Harry F. Byrd Sr, who had controlled the state's politics for decades.

Democrats, who have struggled unsuccessfully to find any candidate to run, said they would welcome Byrd's entry into the race, claiming that his candidacy could only hurt Trible and help a moderate Democrat. "It makes the Democratic nomination worth pure gold," said State Del. Warren Stambaugh of Arlington. "Wonderful! The ball is in the Republicans court now."

The senator could not be reached for comment. A spokesman at his Washington office, which on Thursday had said that Byrd was not reassessing his retirement plans, would not discuss the Godwin statement.

Trible issued a statement thinly disguising his hope that the senator would stick to his decision to retire. Noting that Smith, Godwin and Abbitt are old friends of Byrd, Trible said: "It is only natural that their long-term relationships with him would prompt them to make this request.

"Sen. Byrd is an extremely thoughtful individual who has a reputation for weighing the facts carefully before reaching a decision. I am certain that he weighed the facts long and hard before he made a decision to retire," he said.

"This has got to send everybody back to their calculators," said state Sen. Joseph Gartlan (D-Fairfax). "It brings into play the arithmetic of a three-way race. If Trible and Byrd divide the conservative vote evenly, there's obviously a chance for someone on the moderate to liberal side to move in between and capture the plurality."

The draft-Byrd movement came on the same day as a Petersburg delegate, Norman Sisisky, with ties to conservatives, announced he would not seek the Democratic nomination and former representative Joseph L. Fisher of Arlington said he would.