Chief Justice Paul Hatfield of the Montana Supreme Court, a 49-year-old Democrat, was named yesterday by Gov. Thomas Judge to complete the term of Sen. Lee Metcalf, who died Jan. 12.

Judge said he conferred with President Carter and Metcalf's widow, Donna, before deciding on Hatfield, who is scheduled to be sworn in today and serve at least until Metcalf's term ends next January.

The governor told a news conference at which he introduced the chief justice as Metcalf's successor that the late senator, who had announced he would not seek re-election, had recommended Hatfield as a possible successor.

"In 1976 Sen. Metcalf returned to the state specifically to support this man's candidacy" in his campaign for chief justice, Judge said.

Hatfield, married and the father of two daughters and a son, was a District Court judge in Montana for 17 years and was elected chief justice last year by more than 100,000 votes - the largest margin in the state's history for that office.

At the news conference, he indicated his political outlook by saying, "I have been increasingly concerned about the continuing infringement of federal activity on the sovereignty of the states and the lives of the people."

He said he also intends to "be my own man."

Hatfield said he did not know whether he would run for election to the Senate after his temporary term expires. Judge, who said he had been urged to name himself senator, responded that he would not run if Hatfield campaigns. But Judge left open the possibility he might seek the office if Hatfield does not run.

Hatfield, Montana's youngest elected District Court judge when he won the post in 1960 at age 31, said there were no strings attached to Judge's appointing him to the Senate.

Metcalf, a Democrat who spent 25 years in Congress and was a champion of conservation, was found dead in his apartment in Helena of an apparent heart attack. He was 66.

He was a member of the Senate Energy and Natural Resources Committee and the Governmental Affairs Committee, and was a key supporter of the strip-mining legislation approved in 1977.

At the time of his death, he was working to classify millions of acres of federal land adjacent to Glacier and Yellowstone national parks as wilderness areas.

Judge said he also conferred with Vice President Mondale and members of Congress before making his choice.

He said he had received numerous letters and calls suggesting he resign and take the Senate seat, and that it was a difficult choice for him.

"While it is no secret that I have always been interested in serving in the U.S. Senate, I discarded that option," he said.