The six-month legal battle over whether freshman U.S. Rep. Richard A. Tonry (D-La.) should hold Louisiana's 1st Congressional District, seat entered its latest round today in the state court here.

Tonry beat former New Orleans City Councilman James A. Moreau in the Oct. 2, 1978, Democratic primary and defeated a Republican and an independent candidate in the November General Election.

However, shortly after the primary, Moreau began charging that he lost because of vote fraud, and a federal investigation has resulted in guilty pleas from 20 election commissioners, who said they stole 432 votes for Tonry. Tonry won the disputed election by 184 votes.

Presiding over the unusual civil trial in this community near New Orleans is state District Court Judge Melvin Shortess of Baton Rouge, who has already heard the case once. In the previous hearing, he upheld Tonry's victory, but in the wake of the commissioners' guilty pleas he agreed to hear the suit again.

The only issue on which Shortess will rule is the nullification of his judgement naming Tonry as the winner. Moreau's attorneys will try to prove that fraud in four precincts, perjury by poll commissioners at the first trial and a scheme to cover up the fraud resulted in a bad judgment.

Moreau also wants to be declared the winner of the Oct. 2 Democratic primary, and he wants a general election reheld, with himself as the Democratic nominee. However, the judge said he has no jurisdiction over these matters, and Moreau has appealed that ruling to the State Supreme Court.

The election is also being investigated by the House Administration Committee, and Rep. Mendel J. Davis (D-S.C.), said an audit of voting records in two parishes (counties) of the district should be complete by next week.

If the judge's ruling goes against Tonry, and if the House were to follow that up by voting to expel him, he would become the first congressman turned out office by his peers on the basis of fraud in a party primary, sources close to the case said.

Since the October primary, the Tonry-Moreau dispute has gone through a series of courts. In a news conference this week, Tonry estimated it has been heard by 81 judges.

Tonry has countered the allegations against him with charges of vote fraud in favor of Moreau, and he said the number of votes reportedly stolen in favor of the former city councilman outnumber those allegedly stole for him. Davis said he is investigating this allegation.

In a deposition last week, Daniel Marshall, a St. Bernard Parish poll commissioner, said the congressman voted for himself illegally Oct. 2 in a precinct where he was not registered.

"Tonry came in the main entrance," said Marshall. "The curtain [of a voting machine] was closed, and the bell rang, and he came out."

Marshall said Tonry "had to have voted or the bell wouldn't have rung."

Marshall, one of the poll commissioners who had pleaded guilty to vote fraud, said he covered up Tonry's fraudulent vote by using a name from a list of registered voters in that precinct who were not expected to vote that day.

After the election, Marshall said, Tonry told him "that we wouldn't have to worry about anything."

He also said Tonry approved of his decision to plead the Fifth Amendment and refuse to testify if called during the original trial of Moreau's suit.

Marshall made similar allegations in an affidavit Moreau's attorneys have filed with the House Administration Committee. Tonry denied Marshall's allegations under oath on Saturday.

The House investigation is being aided by the FBI and the General Accounting Office.