The Justice Department yesterday took aim at one of the last bulwarks of southern politics, filing suit to overturn a Georgia law requiring candidates to receive a majority of the vote to win primary or general elections.

The suit, filed under the 1965 Voting Rights Act, could have profound political repercussions throughout the South, according to voting rights specialists and political analysts. Eight other states have similar laws, mandating primary or general election runoffs when no candidate has received more than 50 percent of the vote.

The result, civil rights leaders have long argued, has been to prevent blacks from winning in statewide races or in localities where they do not constitute a majority of the population. In filing its lawsuit yesterday, the Justice Department adopted that argument for the first time, charging that because "voting in the state of Georgia is racially polarized" and "white voters rarely vote for black candidates," runoff elections have discriminated against the state's black minority.

"I view this law as really an electoral steroid for white candidates," said John Dunne, assistant attorney general for civil rights. "It gives white candidates an extraordinary power. . . . It has had a demonstrably chilling effect on the intent and willingness of blacks to be candidates for office."

"This could change the calculus of electoral politics throughout the South," said Frank R. Parker, director of the voting rights project of the Lawyers Committee for Civil Rights Under Law. "There's been election after election in every southern state where a black candidate has won a plurality and then been thrown into a runoff" only to lose to the white candidate. "It's a terrific step for the department to take."

But some political analysts suggested yesterday that if the suit is successful the real beneficiary will be the Republican Party -- a point that was echoed by some Georgia Democrats. Under this scenario, more blacks would likely win Democratic primaries if there is no runoff, but then lose general elections to Republican candidates backed by white "cross-over" voters.

"It makes you wonder why they are doing this," said Georgia Democratic Party Chairman John Henry Anderson. "Why would this become such a pressing issue all of a sudden? . . . The public is not served well when somebody who is not a clear choice of the people is the nominee."

Georgia Assistant Attorney General Perry Michael said the state rejected the department's position that the law was discriminatory, but declined to comment further. Sen. Sam Nunn (D-Ga.), who won his Senate seat after a 1972 runoff election, also criticized the suit, saying that when "you deprive the people of the right to make that decision by majority votes, you've greatly damaged the election process."

The lawsuit comes only two days after a closely watched Georgia gubernatorial primary in which the runoff requirement apparently had no effect on the ultimate outcome. Making his bid to become the state's first black governor, former Atlanta mayor Andrew Young was defeated by Lt. Gov. Zell Miller in a runoff by nearly 2 to 1. Miller, who is white, had also easily outdistanced Young in a crowded field during last month's initial round of voting.

But Justice Department officials contended that in many other instances in Georgia, blacks have been repeatedly kept out of office by "white bloc" voting in electoral runoffs, especially in small rural communities. The majority-vote requirement applies to all local and state elections except the general election for governor.

In 35 instances in recent years, black candidates have come in first in initial primary elections, only to be defeated in the runoff, according to department officials.

The result, Dunne said, is that while blacks constitute more than 25 percent of the state's population, they hold less than 10 percent of state and local offices.

But another major reason for filing the suit, Dunne said, was that department lawyers recently came across "hard evidence" that the majority vote law was enacted in 1964 for the specific purpose of diluting black voting power.

Dunne said the suit had its origins earlier this year when the civil rights division rejected Georgia's application for "pre-clearance" or approval of a change in its laws governing the election of judges. Under the Voting Rights Act, such approvals are required when any one of seven southern states seeks to change its election laws.

In the course of investigating that case, Dunne said, the department came across statements by Denmark Groover, a Georgia legislator who sponsored the 1964 majority vote law, showing that he clearly intended it to dilute the voting strength of blacks. Dunne said Groover repeated similar comments in a deposition 20 years later, and said this showed that the law had been passed with the intent of "keeping blacks down."

A similar lawsuit against the state's law was filed in May by the American Civil Liberties Union on behalf of a black Georgia legislator. Dunne said he expects the issue to wind up in the Supreme Court, resulting in a ruling that would affect majority-vote laws in other states as well.

Other states with similar runoff systems are Alabama, Arkansas, Florida, Louisiana, Mississippi, Oklahoma, South Carolina and Texas.

Among those applauding the department's move was Jesse L. Jackson, who made what he said are the discriminatory effects of primary runoffs a principal issue during his 1984 presidential campaign.

"I feel some relief . . . every now and then you do get a breakthrough," Jackson said yesterday.