The American Civil Liberties Union, departing from its customary neutrality in judicial selections, called yesterday for the Senate to reject President Reagan's nomination of Judge Robert H. Bork to the Supreme Court.

"Judge Bork is, in fact, more radical than conservative," ACLU President Norman Dorsen said at a news conference. "He is certainly well outside the mainstream of conservative judicial philosophy."

Ira Glasser, the organization's executive director, said Bork is "unfit" to serve on the high court because of his views urging limited constitutional protection for personal privacy, free speech and separation of church and state.

"He believes the highest right in the society is for the majority to impose its moral views on the minority," Glasser said. "Had he been around in the 18th century, he would have been against adding the Bill of Rights to the Constitution."

The ACLU said the White House has not accurately portrayed Bork, a federal appeals court judge here, in comparing him to other conservative jurists. ACLU officials said Bork is an extremist unlike such past conservative justices as John Marshall Harlan, Felix Frankfurther and Lewis F. Powell Jr., whose seat Bork would take on the court.

Only once before has the ACLU taken a position on a Supreme Court nominee. It opposed President Richard M. Nixon's appointment of William H. Rehnquist in 1971. The ACLU did not take a position last year when the Senate confirmed Rehnquist as Reagan's nominee to be chief justice.

Dorsen said the ACLU board of directors voted, 47 to 16, last weekend to break with tradition and take a position on the Bork nomination. The board then voted, 61 to 3, to oppose his confirmation.

Dorsen drew a distinction between Bork and Antonin Scalia, another conservative judge named to the high court last year by Reagan.

Dorsen said Bork's writings and judicial opinions are far more extensive than those of Scalia and reveal a man who believes that the Supreme Court's role does not include protecting individuals against majority rule.

The fact that Bork, by replacing the more moderate Powell, may prove a pivotal vote on the court is not the most important consideration, Dorsen said. "We're looking for the long run," he said.

Dorsen also compared Bork unfavorably to Harlan, a former justice for whom Dorsen worked as a law clerk.

Dorsen said Harlan "eloquently defended privacy and separation of church and state," supporting, for example, a ban on organized prayer in public schools.

"If children from Jewish or Jehovah's Witness families, for example, are upset, Judge Bork thinks their remedy is to leave the classroom," the ACLU said in a statement attacking Bork's views.

Morton Halperin, head of the ACLU's District of Columbia office, said the organization is not playing partisan politics in opposing Bork and said he believes that most senators are undecided on whether to vote for confirmation.

"We intend to let Judge Bork speak for himself, to let Bork be Bork," he said. He said the ACLU will "synthesize and summarize" Bork's statements to help senators understand what the nominee stands for.

"We intend to ring an alarm bell. There is a grave threat here to the Constitution," Halperin said.

The Senate Judiciary Committee is scheduled to begin two weeks of hearings on the nomination Sept. 15. A Senate vote on confirmation would not take place until after the court begins its new term Oct. 5.