BALTIMORE, DEC. 4 -- Citing a crowded schedule and unspecified "personal reasons," prominent Washington defense lawyer Abbe D. Lowell has withdrawn as attorney for convicted former Maryland state senator Clarence M. Mitchell III, who faces trial in four more cases in federal court in early 1988.

In what appeared to be a series of sudden decisions, Clarence Mitchell's brother, state Sen. Michael B. Mitchell, and their mother, Juanita Jackson Mitchell, both lawyers, announced at a sometimes acrimonious court hearing Thursday that they will replace Lowell. They will be joined by a third lawyer, Blaine A. White.

Lowell would not specifically say why his firm pulled out of the Mitchell cases. He said the reasons are included in an affidavit filed with trial Judge Norman P. Ramsey, who has sealed the affidavit.

Clarence Mitchell said the pullout was not triggered by a disagreement over trial strategy or related to any inability to pay Lowell's legal fees, two common reasons for attorney withdrawals. But he added: "We've always been short on funds . . . . We're in the process of raising more."

Clarence Mitchell, convicted along with Michael Mitchell last month of accepting $50,000 to block a congressional investigation of scandal-plagued Wedtech Corp., told reporters that his legal representation will not suffer in Lowell's absence, even though his new attorneys must prepare for his next trial from scratch.

Because of the switch in attorneys, Ramsey postponed Clarence Mitchell's next trial, in which Mitchell faces obstruction of justice charges, from Dec. 14 to Jan. 11.

Ramsey's order came after a hearing Thursday at which his initial reluctance to grant a postponement was angrily challenged by Juanita Mitchell.

The 74-year-old woman, a veteran civil rights litigant, widow of longtime NAACP lobbyist Clarence M. Mitchell Jr. and the first black woman to graduate from the University of Maryland law school, told Ramsey, "If our lawyers can't get enough time to prepare for these cases, I'm concerned about my son's constitutional rights."

She said his upcoming trial already is in an "inflamed climate," stemming from a racially hostile remark by a prospective juror that led to an initial mistrial in the Wedtech case last month.

Attorney White, a former member of the Mitchells' Baltimore law firm, also asked Ramsey for more time to prepare for the next case. "I haven't even read the indictment," he said, adding that he was "shocked" when Lowell recently asked him to fill in for him.

Ramsey said his own heavy trial schedule made it difficult to postpone the Mitchell trial, but ultimately he agreed to it.

The Mitchell brothers, both Democrats, have contended that their prosecutions are part of a Reagan administration effort to discredit black leaders throughout the country. Prosecutors have said the the allegations are not true.

Clarence Mitchell faces trial Jan. 11 on charges of obstructing justice by falsifying documents to hide his business ties with convicted Baltimore narcotics dealer Melvin D. Williams. He faces three additional trials in February, March and April on charges of padding hotel expenses by $12,000 as a state senator in 1983, falsifying information on a government-guaranteed loan for an employe in his real estate business and failing to pay taxes on $101,000 in income in 1984 and 1985.