Baltimore Mayor William Donald Schaefer is making plans to hold a massive political fund-raiser in May that would be designed to raise as much as $1 million and put to rest once and for all any doubts that he intends to seek the Democratic gubernatorial nomination in 1986.

Sources here and in Baltimore today said the fund-raiser would be presented as a salute to the mayor's 30 years in public office and a de facto announcement of his candidacy.

"This is a chilling maneuver," said one Baltimore source close to the mayor. It is designed, he said, to lock in supporters who are under increasing pressure to back the all-but-announced candidacies of the two other major Democratic contenders, House Speaker Benjamin L. Cardin (D-Baltimore) and Attorney General Stephen H. Sachs.

"The objectives would be to raise bucks and keep people in place and remind people who need to be reminded that the mayor is a pretty potent force," said the source. "The uncertainty surrounding the mayor's intentions has left people dangling. You can't wait until the last minute."

Schaefer, now in his 13th year as mayor of Baltimore, has repeatedly demurred when asked whether he will run for governor. But with Sachs and Cardin already running hard and raising significant amounts of money, it is growing increasingly difficult for Schaefer to keep key political and financial supporters neutral.

Word of the fund-raiser filtered throughout Annapolis today, which one source said was a deliberate maneuver by the mayor to send a clear signal to the legislature, which over the next nine weeks will be acting on an ambitious agenda sought by the Baltimore city administration.

Legislators from Baltimore and Prince George's County who are close to the mayor said today they have learned that Schaefer will formally declare his candidacy in May, but other sources said the fund-raiser would be used to make Schaefer's intentions clear without the need for a formal declaration at that time.

In either case, the candidate who is expected to suffer the most from a Schaefer candidacy is Cardin, whose political and financial bases overlap the mayor's. Cardin last Sunday held a Baltimore fundraiser that netted more than $250,000, bringing his total funds to about a half million dollars. Sachs has raised a similar amount.