The first black to a mount a serious campaign for mayor of this city is challenging the son of an Italian immigrant in a runoff election Saturday.

Ernest N. (Dutch) Morial, 48, who finished first in the Oct. 1 mayoral primary, faces City Councilman Joseph V. DiRosa, 60, in a campaign in which race has become an issue of importance second only to that of whether the case is being decided in the courts instead at the ballot box.

The two are fighting to succeed Moon Landrieu, a former president of the U.S. Conference of Mayors who has been in office since 1969 and is barred by city law from seeking a third term.

The suits, which questioned Morial's qualifications for office, were finally resolved in his favor Wednesday, but political observers here ask whether the legal questions raised will linger in the minds of the electorate.

The first black graduate of Louisiana State University Law School, Morial was also the state's first black member of the legislature in the 20th century. Appointed a juvenile judge in 1970 by the then-Gov. John J. Mc-Keithen, Morial won a 1972 election for a spot on the state's Fourth Circuit Court of Appeal, making him the highest elected black in the state.

Morial was a judge when he qualified for mayor last summer, and state law bars judges from running for non-judical posts. However, before he qualified, Morial won an injunction from U.S. District Court Judge Fred J.Cassibry setting that law aside.

The state Judiciary Commission appealed that action, but the Fifth U.S. Circuit Court of Appeals did nothing with the case until Nov. 3. That's when it lifted the injunction and the lawsuits were on.

In the furor that followed, Morial resigned his judgeship, but state Sen. Nat Kiefer filed suit, claiming that the appeals court action meant that Morial never was a qualified candidate. Kiefer, who had lost a run-off berth to RiRosa by 265votes in an 11-man field , lost at each level of the state court system, concluding with a state Supreme Court rebuff Wednesday.

DiRosa employed the same argument when he sued to be declared winner of the runoff before the election is not merely based on the litigation.

Already, a record 1,900 absentee balots have been case in the runoff,and up to 83 per cent of the city's 228,735 registered voters are expected to go to the polls Saturday. Of this total, 131,728 are white, and 97,007 are black.

In the first primary, 58 per cent of the black vote went to Morial. In the runoff, he is also counting on support from yonng white professionals and neighborhoods surrounding the city's colleges.

DiRosa is banking on ethnic votes, which in New Orleans includes that of the French Quarter and support from subdivisions that are primarily conservative, white and expensive.

In the campaign. DiRosa is also trying to cut into Morial's support by exploiting the issue of race, an issue each candidate had pledged he would not exploit.

DiRosa told a reporter that Morial was importing armies of blacks into city to register to vote for Morial, and in that same session he referred to blacks as "jungle bunnies." Morial denied DiRosa's charge, and DiRosa denied making the statement, but the reporter and a witness stood by the story.

DiRosa also lashed out at U.N. Ambassador Andrew Young, a native New Orleanian, who was invited here during the campaign to accept an honorary degree from predominantly black Dillard University. While he was here, young said the city was "on trial" in the mayor's election and announced his support for Morial.

DiRosa fired back with a full-page newspaper advertisement in the form of a open letter, in which he urged "outsiders" like young, whose parents still live here, to stay away.

And in a forum before an audience that was approximaately 70 per cent black, DiRosa said Morial's election "would be the most divisivething to hit this city since God knows when."