Former Alexandria vice mayor James P. Moran said he will announce today that he is challenging Democratic Mayor Charles E. Beatley in the city's May 7 elections.

Moran, a two-term Democratic council member who resigned as part of a plea bargaining agreement in a conflict of interest case, must run against Beatley as an independent. Beatley already has won the party's nomination without opposition.

Even so, the former vice mayor, who gained the largest number of votes in the 1982 council election, is likely to win significant Democratic support, several Alexandria politicans said yesterday.

The prospect of a splintered Democratic Party had some Republicans questioning whether they should run. John Williams, Alexandria Taxpayers' Alliance president, is the only other mayoral candidate and also will run as an independent.

"It's a very good time for Republicans," said School Board Chairman Lou Cook, who had been considered a possible Republican challenger to Beatley. She declined yesterday to run, but said the GOP should field a strong candidate in a race with more than two candidates. "With Moran and Beatley slugging it out, if we had a good third candidate we'd have a great chance," she said.

Alexandria Democratic Party Chairwoman Joanne Miller estimated that about one-sixth of the party's central committee would resign to join Moran's campaign. "It's hard to make a damage estimate," she said, "but I see no reason Beatley won't survive."

Republican Party Chairwoman Jane Ring said yesterday that a Republican mayoral candidate may yet emerge today, the last day to file for the election. Peter Straub, an Alexandria lawyer and longtime Republican activist, said he was considering the race because "the squabbling I have seen among city officials recently is an embarrassment."

"Beatley was considered unbeatable until a few months ago," said Cook. But since Democratic council member Donald C. Casey began leading the chorus of criticism against Public Safety Director Charles E. Strobel, Beatley has been perceived as vulnerable, Cook said.

A grand jury report released last week said Strobel had not improperly halted a 1984 drug investigation and said that "baseless" allegations about his performance as head of the city's police and fire departments were fueled by the "politically motivated activities of certain members of the Alexandria City Council."

Moran continued yesterday to blame the mayor for spreading the "scandal" about Strobel and said that because Beatley would not apologize for his "mistake in judgment" he would run against him.

"Though recent incidents have wounded Beatley, he's still a tough, formidable competitor," said state Sen. Wiley F. Mitchell (R-Alexandria). The state senator said Moran was "grossly underestimating" the impact of the the misdeamnor conflict-of-interest charge that forced him to resign from the council last June.

Moran said he was aware Beatley would "play that charge for all it's worth," but felt confident he can defeat the five-term mayor.

Beatley has said he welcomes Moran's opposition. He was in California yesterday and could not be reached for comment.