WASHINGTON (AP) _ Supreme Court nominee David Souter said today that he opposes a flat ban on the death penalty and that the Constitution does not bar its use.

It was Souter's 51st birthday and his third day of testimony before the Senate Judiciary Committee.

Asked if the Constitution's ban on cruel and unusual punishment barred use of the death penalty, Souter replied, ''I think that would be an insupportable constitutional conclusion.''

He said he ''could not join'' any opinion that banned all capital punishment.

If confirmed, Souter's view would be in line with the current majority of the high court. His response underscored his reputation as a law-and-order judge.

Souter explained his approach to a range of criminal law cases, including one in which he supported police use of sobriety checkpoints.

During his tenure on the New Hampshire Supreme Court, it ruled against the checkpoints as extending police action too far in violation of people's rights. But Souter alone dissented.

In his testimony today, Souter said the checkpoints differed from other searches because driving a car is ''a highly dangerous and regulated activity.'' He said if the checkpoints were properly conducted, police were correct in using them to try to prevent tragedy.

The use of police sobriety checks was upheld by the U.S. Supreme Court earlier this year. The court, acting on a Michigan case, ruled 6-3 that such checkpoints do not violate the Constitution.

On another issue, Souter was quizzed about his tough prosecution of anti- nuclear protesters when he was New Hampshire's attorney general - and whether there was an appearance of impropriety because the builder of the Seabrook nuclear plant was helping defray the law enforcement costs.

Souter said he wasn't aware of the donations at the time and had not solicited them. But in retrospect he said he should have insisted the money be returned.

''I think that would have been an appropriate position to take and I wish I had taken it,'' he said.

Today's hearing started with birthday greetings for the New Hampshire judge, now 51.

''Happy Birthday. We didn't bake a cake, but perhaps we'll let you go home after today,'' said Sen. Orrin Hatch, R-Utah, who complimented Souter for his handling of the committee's questioning so far.

At the opening of the televised hearings last Thursday, Sen. Edward M. Kennedy, D-Mass., said, ''the burden of proof rests with those who support a nomination.''

But the conduct of the hearings made clear the opposite was true - the burden was on potential opponents to find some chink in Souter's Yankee armor.

In two days of sparring with the committee last week, Souter skillfully ducked making specific responses on politically difficult issues, especially abortion. On that one, he said he hadn't made up his mind and had made no promises to anyone.

Key committee members, including Chairman Joseph Biden, D-Del., said they planned to take another run at the privacy issue, which is at the heart of the abortion debate, and at others to see if there is a reason for opposing the nomination.

Outside of the committee room, feminist groups on Friday announced their opposition because Souter would not state he would support abortion rights on the court. But so far no committee members have committed themselves to voting against him.

Kennedy said Friday he was troubled by some of Souter's responses to questions, including one where the nominee offered what sounded like a callous explanation.

Kennedy asked about a statement attributed to Souter that said it ''dilutes the votes of those people who can read'' if illiterates are allowed to vote. Souter said the remark was ''kind of a statement of math.''

Souter's response stood out in part because the bookish bachelor has taken great pains in the hearings to portray himself as compassionate and aware of a broad range of human concerns.

After it completes it hearings, the committee will make its recommendation to the full Senate.

Souter's supporters have expressed great confidence he will join the court this fall.

Souter himself has been cautious, however.

Sen. Warren Rudman, R-N.H., a longtime friend of Souter's who is playing host to him in Washington during the nomination process, said the judge won't even let people discuss possible places to live if he moves to the capital.

''Maybe he's superstitious,'' Rudman said.