Justice Department officials have failed for the second straight time to win a conviction in a group of cases in which they have charged black civil rights activists in Alabama with voting fraud.

A federal judge in Birmingham declared a mistrial last Wednesday in the case of James Colvin, mayor of the small town of Union, after one black juror held out for acquittal through eight hours of deliberations.

This followed a case in July in which Albert Turner, a former aide to the Rev. Martin Luther King Jr., and two other defendants were acquitted of illegally tampering with absentee ballots.

These charges, along with pending cases against four other civil rights organizers in Alabama's rural Black Belt, have sparked accusations that the Reagan administration is conducting a "witch hunt" aimed at intimidating black voters.

A group of Alabama blacks has struck back with a civil suit that charges the Justice Department with targeting prominent black leaders for indictment, while ignoring repeated allegations of absentee-ballot fraud by whites. The suit was dismissed last week on procedural grounds.

"These are selective prosecutions," said Randall Williams of the Southern Poverty Law Center, which brought the lawsuit. "There was never any investigation of this when it was the white folks who were suspected of doing the ballot-stuffing."

"They have gone out of their way to get a key black civil rights leader in each of these counties," said state Sen. Hank Sanders, who is aiding the defendants.

Black activists say the Justice Department has jumped in at the behest of southern Republicans, rather than deferring to local authorities as it has in the past. They point to the Justice Department's announcement last year that all 93 U.S. attorneys were joining in "a comprehensive program to combat voter fraud."

Justice Department spokesman Terry H. Eastland said the prosecutions were in no way politically motivated or designed to harass black voters. "It's very odd to see these civil rights attorneys who want 'local option' when voting rights are at stake," he said.

Eastland also dismissed suggestions that federal prosecutors are failing to pursue similar offenses by whites.

"We have not had a complaint by blacks against the white establishment . . . that is specific enough to enable us to investigate," Eastland said.

Black leaders say they have made several such specific complaints.

Eastland said the department has secured voting fraud convictions against "a tremendous number of white defendants" in the last two years, including one in Alabama, three in Georgia and 30 in North Carolina.

While political analysts debate whether the controversy will affect such contests as Republican Sen. Jeremiah Denton's 1986 reelection campaign, the real stakes in southwestern Alabama are control of the county commissions and school boards.

Although black turnout increased after passage of the 1965 Voting Rights Act, many white candidates continued to win elections on the strength of a few hundred absentee ballots, which are crucial in county contests that rarely draw more than 5,000 voters.

"In the late 1960s, you'd find 250 to 300 votes in the absentee box, all of them for the white candidate," said Jack Drake, a white Democratic attorney in Tuscaloosa. "When you examined them, you'd find people who lived in Atlanta or Nashville who had been gone from the county for 20 years."

In the late 1970s, however, black activists mastered the art of collecting absentee ballots from the elderly and people who worked outside the area or had moved away. By 1982, blacks had wrested control of the political machinery in five of the 10 impoverished Black Belt counties, enabling them to hand out courthouse jobs that had long been dominated by whites.

Albert Turner, who marched alongside King at Selma in 1965, is one of the civil rights leaders who has vigorously solicited absentee ballots. In 1982, Turner ran unsuccessfully for probate judge in Perry County. Afterward, county District Attorney Roy Johnson sought to indict Turner for voting fraud. Johnson said he examined the absentee ballots and found that "Turner wrote his own name in after he scratched out what these people wrote."

But the local grand jury simply issued a report calling for an outside probe of voting irregularities. "Our grand jury was predominantly black," Johnson said. "They have known these people Turner and his colleagues all their lives."

Johnson gave his findings to the Justice Department's Criminal Division, and when Turner started gathering absentee ballots from elderly voters and nursing home residents for the September 1984 Democratic primary, investigators were ready. The night before the primary, when Turner and colleague Spencer Hogue mailed 504 absentee ballots, FBI agents were stationed inside the county post office, numbering the ballots as they dropped through the chute.

The FBI interviewed 75 people whose ballots appeared altered. At least 25 allegedly said their ballots did not reflect the way they voted, and they were bused 200 miles to testify before a federal grand jury in Mobile. Turner; his wife, Evelyn; and Hogue were indicted on charges of voting fraud.

Civil rights lawyers charge that the FBI deliberately intimidated many witnesses. "A number of them have said to me they do not intend to vote again," Sanders said.

"When you're dealing with someone 70 or 80 years old, and you're the FBI, and you go to their house and say you may have committed a crime, they will tell you anything you want to hear," Sanders said.

Turner did not deny that he urged elderly voters to change their ballots and sometimes made the changes himself, but said he did nothing illegal because he had their permission.

At the trial, several elderly witnesses backed off from what they had told the FBI, and a jury of seven blacks and five whites acquitted the defendants.

Jeff Sessions, the Republican U.S. attorney in Mobile, said 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."

The case against Colvin, who works for the Greene County tax assessor and is mayor of a town of 373 people, turned on 12 absentee ballots cast by county residents for their out-of-state relatives.

Prosecutors charged that Colvin solicited the absentee votes on behalf of a candidate he supported, picked up some of the ballots and arranged for them to be mailed. Colvin said he told the residents that their relatives had to approve the ballots cast in their name; some witnesses said they had obtained such permission and others said they had not.

Colvin's lawyers produced experts who testified that such "proxy voting" was a longtime practice in the county, and that it was unclear whether it is illegal under state law.

The defense also charged FBI intimidation. One witness who cast a proxy ballot testified that an FBI agent told him that he could receive a long prison term for his offense, but that he would be placed on probation if he cooperated. Defense lawyers said the witness later told them he was under the mistaken impression that he had been charged in the case.

Colvin, who faces retrial in November, called the charges politically motivated and praised one of the two black jurors for holding out for acquittal. "I'm happy that he had the courage to hold out and not convict me for something I didn't do," he said.

The federal investigations, which are continuing, have produced five more indictments in Greene County. Spiver Gordon, a leader of the Southern Christian Leadership Conference, is to be tried later this month. Bobbie Simpson, a deputy county registrar and the only white defendant, went on trial last week.