A Republican senator is seeking a judicial inquiry into whether it was improper for a senior judge on the 7th U.S. Circuit Court of Appeals in Chicago to write a letter opposing the nomination of Daniel A. Manion to that court.

The unusual letter was written by Judge Luther E. Swygert, according to an article to be published Monday in The National Law Journal. Swygert's letter was sent to Sen. Paul Simon (D-Ill.), a leading Manion opponent, who read it, without identifying Swygert by name, during Wednesday's Senate debate on Manion.

Manion, a South Bend, Ind., attorney, was confirmed by the Senate Wednesday on a 50-to-49 procedural vote in which a symbolic tie-breaking vote was cast by Vice President Bush. Manion's confirmation came after a long and bitter debate over whether he is too inexperienced for the federal bench.

Sen. Orrin G. Hatch (R-Utah) questioned during the debate whether it was improper for a federal judge to use federal funds to lobby Congress and whether writing the letter violated judicial ethics.

A spokesman said yesterday that Hatch has drafted a letter to the chief judge of the 7th Circuit to seek a judicial review of the incident. Such a complaint could trigger a formal disciplinary proceeding against Swygert, 81. Hatch had earlier said he might seek an FBI investigation, but now thinks that a judicial review would be more appropriate, the spokesman said.

Swygert, who was nominated to the appeals court by President John F. Kennedy, said in his letter to Simon that Manion's appointment would be "a great disservice to the 7th Circuit and to our entire judicial system."

A law clerk to Swygert said yesterday that the judge would have no comment. But another 7th Circuit judge, William J. Bauer, told The National Law Journal that Swygert has acknowledged writing the controversial letter.

"He felt strongly about this," Bauer said. "He has strong feelings about this court." Bauer said he sees no legal or ethical violation, but that "whether this was an appropriate thing to do is a different matter."

A Simon aide said judges frequently write to members of Congress on pending issues, including nominations. "It was a personal letter to Paul Simon," the aide said. "I don't think it's illegal, and it's certainly not uncommon."