Liberals and civil rights activists are kicking off their 1986 campaign against President Reagan's judicial nominees with an assault on two who they say have tried to chill minority voting rights.

Jefferson B. Sessions, the U.S. attorney in Mobile, Ala., has been the focus of considerable controversy for his unsuccessful prosecution of three black civil rights activists on voting-fraud charges. Opponents also have mobilized for a hearing today on Sidney A. Fitzwater, a state district judge in Texas, for his role in a 1982 sign-posting incident.

Both nominations died last year in the Senate, which has been a frequent battleground over Reagan's effort to fill the federal bench with conservatives. The Alliance for Justice, a coalition that includes the NAACP Legal Defense Fund, People for the American Way, and the Leadership Conference on Civil Rights, continues to lead the criticism through its Judicial Selection Project.

Such openly partisan groups as Americans for Democratic Action, sensing a ripe political issue, are taking a more active role in fighting Reagan's judicial nominees. The Congressional Black Caucus voted last week to oppose Fitzwater and Sessions. Nevertheless, Senate Republicans are confident that both will be confirmed.

Opposition to Fitzwater, 32, who was recommended to the White House by Sen. Phil Gramm (R-Tex.), centers on his posting of signs in minority precincts in Dallas the night before the 1982 election. The 24-foot signs warned voters that "You Can Be Imprisoned" for violating the Texas election law, adding: "Don't Risk It, Obey the Law."

Fitzwater testified at his brief Senate confirmation hearing in November that all the signs' statements were in accordance with the law. But critics say there is nothing in Texas law that makes it a crime, as the signs said, to "influence or try to influence a voter how to vote."

"I would hope a sitting judge would know that the First Amendment applies in Texas," said Nancy B. Broff, director of the Judicial Selection Project.

Fitzwater said that another Republican judge asked him to post the signs in three precincts and that he told Fitzwater it was an officially authorized ballot security program. Fitzwater testified in a civil suit that he did not study the signs and drew no conclusion from the fact that he was asked to post them only in black areas of south Dallas.

The Justice Department later found fault with the sign-posting, and Assistant Attorney General William Bradford Reynolds said he was "concerned that no nonracial justification has been offered for placing most of the signs at minority precincts."

Ann Lewis, director of Americans for Democratic Action, said that even if Fitzwater "unthinkingly hung the signs in a minority precinct without stopping to question whether the language was really accurate, surely in the last three years he'd reflect on that . . . . I don't think he's got the common sense to be a federal judge."

Willie Velasquez, director of the Southwest Voter Registration Education Project in Texas, said, "I feel certain that the posting of these red-and-black signs in a black precinct was designed to diminish the black vote. That is a shameful and disgraceful thing for a sitting judge to do."

Sessions attracted national attention last fall when he unsuccessfully prosecuted Albert Turner, once an aide to the Rev. Martin Luther King Jr., and two other defendants on charges of illegally tampering with black voters' absentee ballots. Critics accused Sessions of selective prosecution, saying that he brought no charges against whites repeatedly accused of similar ballot fraud in Alabama's rural "Black Belt" counties.

Defense lawyers charged that the Federal Bureau of Investigation intimidated some elderly black witnesses, busing them 200 miles to testify before a grand jury. The case fell apart when some of these witnesses changed their stories at the trial.

Sessions said last fall that Turner "put on a brilliant defense and whipped us fair and square in the courtroom, but I guarantee you there was sufficient evidence for a conviction." He said he would "prosecute anybody, white or black" and that he had been given no evidence of voting fraud by whites.

The administration is also considering a federal judgeship for Samuel T. Currin, the U.S. attorney in Raleigh, N.C. Currin, a former aide to Sen. Jesse Helms (R-N.C.), issued well-publicized warnings about prosecuting voting fraud during the 1984 campaign that state elections officials criticized as inaccurate.

"It is nothing short of scandalous that Currin's office would issue so faulty a memorandum, a document clearly aimed at quashing legal Election Day activity," the (Raleigh) News and Observer said in a 1984 editorial.