It was only a month ago that Francis T. West, the former mayor of Martinsville, Va., arranged for Rep. Paul S. Trible Jr., the apparent Republican nominee for the Senate, to meet about a dozen political and business associates.

Two weeks later, West followed up with a letter, telling the group he was contributing $500 to the Trible campaign and inviting them to do the same. On paper, it looked as though Trible, a 35-year-old congressman from Newport News, had won the support of a key member of the so-called "coalition," a loose group of conservative Virginians who for years have crossed party lines looking for suitable candidates.

But that has changed since West heard that Sen. Harry F. Byrd Jr., the Senate's only independent, might run again for the seat held by his family for almost 50 years. In West's view, there is no contest if Byrd is in the race.

"Man and boy, every vote in any senatorial contest I have ever cast has been for a Byrd," said West, 62, a member of the Martinsville City Council and president-elect of the Virginia Municipal League. "I wish to cause Paul Trible no discomfort but the Byrds are on a special shelf by themselves in Virginia."

That loyalty was echoed by other members of the "coalition" yesterday, most of whom had signed onto the draft-Byrd effort as soon as it was officially launched Friday by former governor Mills E. Godwin, former state delegate W. Roy Smith and former congressman Watkins Abbitt.

Some, like West, are former Democrats-turned-Independent who have consistently supported Republicans. Others, like Smith and Abbitt, had been supporters of Gov. Charles S. Robb, the first Democrat to be elected governor in Virginia in 16 years.

Former state legislator D. French Slaughter of Culpeper, who briefly considered a challenge to Trible for the Republican nomination, said yesterday he has canceled plans to go to the GOP convention in Richmond this weekend. "I told my local chairman I could not go as long as there was a possibility that Sen. Byrd might be a candidate," said Slaughter who left the Democratic party in 1973 and became a Republican in 1980.

"These men, all of them friends for decades, see a last chance to keep their old friend in the Senate," explained Edward DeBolt of Arlington, campaign coordinator for Byrd in 1976 and a Republican strategist. "It's like calling on Al Jolson to sing one more song."

As of early yesterday, the petition drive reportedly had collected nearly one-third of the almost 11,000 required for Byrd to qualify as an independent candidate by next Tuesday. Byrd's Washington office reported a stream of Virginians calling in to offer their support. And several Byrd loyalists expressed confidence that their man would run.

"If Sen. Byrd didn't have a pretty good feeling that he could win the election, he wouldn't consent to our circulating these petitions," said J. Smith Ferebee, a 76-year-old insurance executive who not long ago pledged $1,000 to Trible.

Ferebee, who talked to Byrd Monday night, said the 67-year-old senator, who said last November that 18 years in the U.S. Senate was long enough, no longer has to weigh any objections his wife Gretchen might have to another campaign. "Gretchen was the first one to go to him and release him because she knows how important it is," said Ferebee.

The Byrd effort has caused considerable consternation in the Trible campaign which, for months, has actively, and to some extent successfully, wooed members of the coalition. "If Sen. Byrd decides to run, we will have to redesign our fund-raising and place more emphasis on political action groups and a greater number of small donations," Trible campaign manager Judy Peachee told The Associated Press. "It probably will cost us $200,000 to $300,000."

Trible, appearing at a Rotarian lunch in Northern Virginia yesterday, said the threat of a Byrd candidacy, which he learned about 30 minutes before the official announcement Friday, has not affected his commitment to the race. "I will stay the course," he told reporters.

Trible, who began campaigning for the Senate before Byrd announced his retirement, said he has not encountered any resentment from Byrd loyalists. "I think that reflects the caution and consideration for the senator that I showed the extra mile, in having the consideration to let him make his decision on his own timetable and for his own good reasons," said Trible, who had said all last year he would not run for the seat if Byrd sought reelection.

Leaders in both parties are pondering the various mathematical equations that would be set in motion by a three-way Senate race. But in a year of political uncertainties and constant surprises, few were willing to make predictions about the outcome of what is expected to be a highly volatile week. Democrats have no apparent front-runner for their nomination, to be decided this weekend.

"In terms of the longest, furthest reaching impact on both parties, this is probably going to be the most tumultuous and meaningful week in Virginia politics in years," said DeBolt.