Joseph L. Fisher, a former Democratic congressman from Arlington, is expected to join the race for his party's Senate nomination in Roanoke today, seven days before several thousand Virginia Democrats arrive there for what is likely to be a bloody convention.

"Joe is going to announce his intention to seek the nomination," said supporter Christopher Spanos, one of several Northern Virginia Democrats who have been organizing the Fisher campaign.

Fisher, 68, decided last winter against trying to regain his 10th District congressional seat and joined Gov. Charles S. Robb's cabinet in Richmond as secretary of human resources. He will become the third serious candidate and the second from Northern Virginia to seek the Democratic nomination, a prize that some party officials fear lost its appeal after several ranking state officials decided to shun it.

Fairfax County prosecutor Robert F. Horan Jr., 49, declared his candidacy last weekend, becoming the first to break the deadlock in the party. State Sen. Virgil Goode, 35, of Rocky Mount has since announced his entry in the race.

Several Northern Virginia Democrats expressed a fear Thursday that Fisher and Horan will divide the region's 689 delegates, lessening the chances that either can win the nomination and diluting suburban Washington's bargaining strength at the convention.

"It certainly will create a problem," said Dottie Shick, Fairfax County Democratic chairman, with a deep sigh. "There are many people who are loyal to both . . . and they are suffering."

The split between Fisher and Horan supporters is along both philosophical and regional lines--pitting the liberal former three-term congressman from Arlington against Fairfax's four-term prosecutor known best for his unflinching stances on law-and-order issues.

Several observers have said that Fisher, who lost his seat to Rep. Frank Wolf in 1980, may be too liberal for Virginia's conservative electorate, but his supporters argued Thursday that the former congressman has the age and experience to oppose Rep. Paul S. Trible, the 35-year-old likely Republican nominee.

Virginia Democrats, stung by a series of traumatic incidents that have left the party in disarray, were treated Thursday to new speculation that Lt. Gov. Richard J. Davis might yet consider accepting a draft nomination and that Sen. Harry F. Byrd Jr. may reconsider his announced retirement.

Davis, the favorite among party regulars who said last week he would neither run nor accept a draft, told reporters in Charlottesville that he might become a candidate if the Roanoke convention becomes hopelessly deadlocked. But Davis issued a clarifying statement Thursday that stressed that "it is not my plan to accept a draft."

Jack Davis, Byrd's administrative aide, confirmed Thursday that the 67-year-old senator had received numerous calls and letters from supporters urging him to run for reelection. "We've had a considerable number of calls, particularly in the last couple of days, from people who seem to be disturbed by the unsettledness," said Davis, "and they all end up saying they hope he will reconsider."

Davis and Sen. Elmon Gray of Sussex, a close Byrd supporter, dismissed any notion that Byrd is reconsidering. "I think the senator, once he made his decision, is entitled to a fine retirement," said Gray who said he hadn't heard the reports "anywhere where I'd pay any attention."

Meanwhile Thursday, Peter Tsakanikas, a 50-year-old Rosslyn businessman who first announced for the Senate last week, officially entered the race with an attack on "welfare for the rich." Tsakanikas ran in the 1970 Republican gubernatorial primary in Maryland under the name Peter James and lost.

Also declared as a candidate for the Democratic nomination is Del. Floyd Bagley of Prince William.