Until last Friday, Alexandria Mayor Charles E. Beatley's biggest worry was having too much money for his May 7 reelection campaign.

He was gliding unopposed toward his sixth three-year term as mayor when two candidates announced the unexpected: they would challenge the "unbeatable Beatley."

First, James P. Moran, the former vice mayor who resigned last June while facing a conflict-of-interest charge, challenged Beatley. Moran, once regarded as Beatley's protege, held a news conference Thursday to demand that that if the mayor did not apologize to Public Safety Director Charles T. Strobel, he, Moran, would run as an independent against his fellow Democrat.

Then, Alexandria Taxpayers Alliance President John D. Williams pulled out of the Republican race for the City Council on Friday and into the mayor's race. His goal: to give "voters a different choice than two liberal Democrats."

"The mayor's in hot water now," said School Board Chairman Lou Cook, who along with a handful of other Republicans last week seriously considered challenging Beatley. Cook said that if the Strobel episode had occurred earlier, more Republicans would have had time to raise the necessary campaign funds, "but two months just isn't much time."

Beatley, who in 1982 soundly defeated Republican Robert L. Calhoun, is a 68-year-old retired airline pilot who is not accustomed to political turbulence. In his last campaign he spent only about $13,000, and he had a few thousand left over.

But now Beatley, who on Feb. 22 could persuade only one other person on the seven-member council to ask the city manager to place Strobel on administrative leave, is finding he is not the only voice in the city.

Moran is echoing the words of last week's special grand jury report, which said Strobel did nothing wrong and lashed out at the "callous, politically motivated" actions of "certain council members" who had attempted to discredit Strobel.

Beatley, who spends so much time at his $12,500 post that some city staff members refer to him as "the full-time mayor in a part-time job," was in California vacationing this week and could not be reached for comment on how much money he might need this time.

Moran said he had $9,000 of the $30,000 war chest he believes he needs to defeat Beatley.

Williams, 47, who is considered an "extremist" by both the Republican and Democratic party chairmen, said he did not now how much money he would raise for the race, but he said the campaign will be "low-cost." When he ran for City Council in the 1984 special election, he gained 17 percent of the vote.

Vice Mayor Patricia S. Ticer, a Democrat, said she believed the challenge will be good for Beatley. "He's been in the doldrums lately, not going to as many meetings and getting out as much. This will get him going."

Beatley has mounted vigorous campaigns before, and his supporters note that when he was summoned from a political retirement in 1979 he easily ousted mayor Frank Mann, who was regarded as a power in Alexandria politics.

Of course the city has changed considerably since then, and that clearly will be a part of the challengers' strategy: to persuade new voters that Beatley is out of touch with them and that his actions have smeared the city's reputation.

"We need leadership . . . . It's time to look to the future," said Moran.

"They're terrified of me because I'm a breath of fresh air," said Williams, who has tangled with city officials in the past over his refusal to pay personal property taxes on his automobile. (He says he now pays his taxes on time.)

In two months the city voters will get their say. And no one, including Beatley, is worrying about what to do with any money that might be left over on May 8.