Three more sharply contrasting senators would be difficult to find: a folksy, southern Democrat who often votes with Republicans; an urbane, northern Republican who often votes with Democrats, and a cautious, western centrist known for balancing each liberal vote with a conservative one.

And yet, Howell Heflin (D-Ala.), Arlen Specter (R-Pa.) and Dennis DeConcini (D-Ariz.) this week have become political brethren: They are the "swing" votes on the Senate Judiciary Committee, and as they go, so could go the nomination of Judge Robert H. Bork to the Supreme Court.

With the 11 other committee members expected to split 6 to 5 against Bork, the judgment of this unlikely trio will decide whether Bork's name goes to the Senate floor with or without the influential committee's blessing.

Even in appearance, they are an unmatched set. DeConcini has the gaunt face of the hard-nosed former prosecutor that he is; Specter, also a former prosecutor, has a studious demeanor, half-glasses poised on his nose, and Heflin, a huge, rumpled figure, resembles a home-grown southern pol of yore.

Each has vowed to keep his mind open until the hearings conclude, but yesterday they began to tip their hands as each took a turn at questioning Bork. Two said they came away with serious doubts about Bork's judicial philosophy.

After a contentious exchange on women's rights, civil rights and privacy, in which Bork sought to emphasize moderate features of his philosophy, DeConcini shook his head and told the nominee that he was not satisfied "as to how . . . you're going to protect the citizens of this country" against sex discrimination.

"I'm not trying to determine whether or not he's a conservative. My tastes actually run in that direction," DeConcini said in an interview afterward. "I'm trying to decide whether or not he is really radical."

Specter, known for his fiery style in his days as Philadelphia's district attorney, put Bork on the stand for shifting his interpretation of what kinds of speech he would protect under the Bill of Rights. Bork told him that a 1971 paper in which he argued that only political speech should be protected was "just wrong," adding that he had repudiated it five years ago.

But Specter also sharply challenged Bork on how he could reconcile his philosophy of "judicial restraint" with the court's role in responding to what Specter called "the needs of the nation" -- rectifying moral wrongs not spelled out in the Constitution.

"You really have to raise a question about his approach," Specter said during a brief break in the questioning.

Heflin, by contrast, went easy on the nominee. Ever the folksy southerner, he asked Bork to recount his personal history, including youthful dalliances with socialism and liberalism, and even asked why he wears a beard.

"There's nothing wrong with it," Heflin drawled with a chuckle. "I want to make that clear because there are a lot of bearded voters out there. We've got some in Alabama."

How the three ultimately will vote is a matter of some intrigue on the committee, and among political analysts. It is widely believed that Heflin, former chief justice of the Alabama Supreme Court, who frequently has supported the White House in close votes, is the most sought-after of the trio. If he opposes Bork, the thinking goes, a number of other southern Democrats will follow suit, invoking the view of this trusted conservative to justify what could be a difficult vote.

But fellow Democrats said yesterday that Heflin is leaning strongly in favor of Bork, his claims of indecision notwithstanding. "I think Heflin is looking for a reason to justify voting for {Bork} and DeConcini is looking for a reason to justify voting against," said a Democrat close to both senators.

In addition, political analysts caution that DeConcini, who faces reelection in 1988 in heavily Republican Arizona, may feel outside pressures to vote for Bork despite his announced concerns. Indeed, the two-term Democrat spent part of an afternoon break yesterday reading home-state newspaper articles evaluating his stance on the nomination.

DeConcini's hard-edged questioning of Bork surprised many committee observers, who have come to know him as a man who guards his low profile. When Bork invoked the example of male and female restrooms to defend his assertion that the sexes could reasonably be treated differently under the Constitution, DeConcini shot back:

"Isn't that a bogus argument? We're not talking about unisex toilets here. We're talking about fundamental rights that women, for too, too long have not been provided."

"That's right," Bork responded, then added: "There is no reason whatsoever in my record to think that I have any problem protecting women or any other group."

Bork took pains to point out to each of the three senators that his judicial philosophy accommodated minorities, women, free speech and other constitutional concerns they raised.

Abandoning his guarded demeanor of Tuesday, the first day of the hearings, Bork yesterday engaged in sometimes pointed repartee with committee members, recounted humanizing anecdotes about himself and explained his judicial philosophy more in laymans' than lawyers' terms.

"I think he helped himself," Heflin said. "I thought {Tuesday} he came across as being awful dull, and I don't think the American people could follow him . . . . Now we've gotten down to where people can understand."

Where that left the trio at day's end was as unclear as it was a week ago. As Heflin put it: "Is he a zealous right-winger with a radical agenda, or is he an evolving individual with a great intellectual curiosity who wants to examine and experience the unusual and the unconventional?"

Asked if this interest in personal issues meant Heflin had no more constitutional concerns about the nominee, the senator gave his customarily cagey response: "Sure I do. It was just getting mighty dull and I thought I ought to perk it up a bit."