Senate Majority Leader Robert C. Byrd (D-W.Va.) threatened yesterday to "slow down" confirmation of a successor to retired Supreme Court Justice Lewis F. Powell Jr. if Republicans continue to block action on major Democratic initiatives, including arms control and campaign spending limits.

While promising not to engage in "deliberate stall for the sake of stall," Byrd said, "It might not be a bad idea to say there are equally important matters facing the Senate . . . . If we're going to have all this stalling by the Republicans, then let's just slow down and take a closer look at this nomination."

Byrd's warning at a luncheon meeting with reporters amounted to an escalation of Democratic efforts to counter GOP filibusters that have stalled action on campaign finance revision and a defense authorization bill that would restrict testing of the president's Strategic Defense Initiative.

Delaying or defeating Reagan's Supreme Court nominee may prove a difficult task, however. Despite warnings from liberal Democrats that the nominee should not upset the court's ideological "balance," the first battle is expected to turn on three moderate-to-conservative lawmakers who hold the balance of power in the Judiciary Committee.

Sens. Howell Heflin (D-Ala.), Dennis DeConcini (D-Ariz.) and Arlen Specter (R-Pa.) have been the key swing votes in past nomination battles in the committee, where Democrats now have an 8-to-6 majority. All three voted to confirm Chief Justice William H. Rehnquist last year, and Heflin was one of two Democrats to vote for controversial Appeals Court Judge Daniel A. Manion.

"My general philosophy is that the president has the right to make appointments unless there is an overriding reason why the nominee should not be confirmed," Heflin said yesterday. While he would be skeptical of someone who is "just an ideologue," he said, "we ought to look, number one, to qualifications."

DeConcini said, "I expect a very conservative nominee. I start with the assumption that the president has a right to appoint who he wants."

DeConcini said that "a filibuster would be broken" and that liberal hopes of beating the nominee on ideological grounds are "just wishful thinking."

Committee Chairman Joseph R. Biden Jr. (D-Del.), meanwhile, has backed away from statements last fall that he would have no problem voting for Appeals Court Judge Robert H. Bork, who sources said yesterday is the leading candidate for Powell's seat.

Biden now says he meant that he would support Bork to replace another conservative justice, but wants to preserve diversity on the court. He told CBS' "Face the Nation" Sunday that Bork is "a brilliant man . . . but it does not mean that there should be six or seven or eight or even five Borks."

Biden, a 1988 Democratic presidential candidate, said the Senate has as much right as the president to insist on "ideological purity." Sen. Paul Simon (D-Ill.), another committee member and presidential contender, said the court must not be an ideological "pendulum."

But Sen. Gordon J. Humphrey (R-N.H.), a conservative Judiciary member, said the coming battle "is all about philosophy, and everyone knows it."

"The president is in a no-lose situation," Humphrey said yesterday. "If Ronald Reagan is looking for an issue to regain the initiative -- and he's dead in the water because of the Iran-contra business -- this is it. He's either going to get a conservative nominee on the court or the Republicans will cream the Democrats in 1988" because of the political fallout.

"Everyone is going to make a big deal about being objective about this, but the truth is that 95 percent of the Judiciary Committee is locked in granite," a Republican Senate aide said. For example, he said, "I think Biden will publicly agonize and then vote against a conservative nominee, even if it's Judge Bork."

At the same time, he said, "The opposition will be scrounging around in the dirt for whatever they can dredge up."

Specter said yesterday, "I think there's a real possibility of a stalemate."

Byrd's threat to slow the confirmation process as leverage on Republicans for action on the Democrats' agenda was coupled with a warning that the White House will face further trouble in the Senate if its nominee is "ideologically rigid and inflexible."

Democratic Whip Alan Cranston (D-Calif.) made a similar point in a letter to other Senate Democrats, urging them to form a "solid phalanx" of opposition if the nominee is an "ideological extremist."

But Byrd drew a distinction between conservatism and ideology, saying that if a conservative nominee is qualified without being "inflexibly wedded to a highly visible straitjacketed position," it "would not be too hard" to win Senate confirmation.

Asked about Bork, Byrd said he was "highly capable" but that he was "not prepared to say" whether Bork is too ideological.

Byrd said Bork might have "some problem" because, as solicitor general in the Nixon administration, he fired Watergate special prosecutor Archibald Cox at President Richard M. Nixon's request. But he also noted the Senate later confirmed Bork for the appeals court here.

Byrd said the administration would be "inviting problems" by nominating Bork, but added later, "I frankly think {Bork} probably would be confirmed."

Liberal activists say they are confident of lining up the 41 votes needed to sustain a filibuster against a conservative nominee, but that is not clear. They could have trouble winning over Democrats who helped Rehnquist win confirmation on a 65-to-33 vote last fall, including Lloyd Bentsen (Tex.), David L. Boren (Okla.), Dale Bumpers (Ark.), Lawton Chiles (Fla.), DeConcini, Alan J. Dixon (Ill.), Heflin, Ernest F. Hollings (S.C.), J. Bennett Johnston (La.), Sam Nunn (Ga.), William Proxmire (Wis.), David H. Pryor (Ark.) and John C. Stennis (Miss.).