A senior Democrat on the Senate Judiciary Committee said yesterday there is "no conceivable way" that confirmation hearings on a successor to Supreme Court Justice Lewis F. Powell Jr. will begin until after the August congressional recess, and that it will be difficult to confirm a new justice before the court's term begins in October.

Sen. Patrick J. Leahy (D-Vt.), head of the panel's judicial-screening unit, said the nomination would require "great scrutiny" and that he hopes President Reagan will resist "the temptation to go with ideology over competence." This, he said, could lead to "a very prolonged battle" that would "damage" an administration weakened by the Iran-contra affair.

But Sen. Strom Thurmond (S.C.), the panel's ranking Republican, urged that the hearings begin before the August break, saying that "the public will not stand for . . . delaying the nomination."

The committee's chairman, Sen. Joseph R. Biden Jr. (D-Del.), said in a statement last night that the nomination could turn on whether Powell's replacement "would alter the balance of the court" and that he would "resist any efforts by this administation to do indirectly what it has failed to do in the Congress -- and that is impose an ideological agenda upon our jurisprudence."

Biden said that "diversity" on the court is important, and that while he does not regret his vote last year to confirm Justice Antonin Scalia, "that does not necessarily mean I would support the nomination of yet another individual whose views are identical or nearly so with those of Justice Scalia."

The vacancy presents a dilemma for Biden, who has been spending much of his time campaigning for the 1988 Democratic presidential nomination. Biden, who has drawn some criticism for being a part-time chairman, will have a prime opportunity for national television exposure, but at the expense of precious time on the hustings in Iowa and New Hampshire.

Biden will also come under strong pressure from liberal groups to block a conservative nominee, regardless of intellectual or legal credentials. But Biden, like many other Senate Democrats, has been reluctant to oppose judicial nominees solely on ideological grounds.

Biden raised the issue himself in an interview with The Philadelphia Inquirer last fall, when he talked about the possible nomination of former solicitor general Robert H. Bork.

"Say the administration sends up Bork and, after our investigation, he looks a lot like another {Associate Justice Antonin} Scalia," Biden said. "I'd have to vote for him. And if the groups tear me apart, that's the medicine I'll have to take. I'm not Teddy Kennedy."

Sen. Edward M. Kennedy (D-Mass.), for his part, said yesterday: "Now the Senate will be watching carefully . . . to ensure that President Reagan does the right thing instead of the far-right thing in filling the large vacancy that Justice Powell leaves."

The panel has scheduled mid-July hearings on two controversial judicial nominees, law professor Bernard Siegan and International Trade Commission member Susan Liebeler.

When Rehnquist and Scalia were nominated on June 17, 1986, then-chairman Thurmond sought quick confirmation hearings and eventually agreed to wait six weeks. The two were confirmed Sept. 17.

The fact that the Democrats plan a prolonged inquiry and will control the Senate confirmation machinery for the first time in Reagan's presidency is one of the ways that the confirmation process will differ from last summer's battle over the nomination of Chief Justice William H. Rehnquist, several observers said.

Senators' distaste for a frontal ideological assault became clear during the Rehnquist debate, which focused on such issues as a restrictive covenant on a deed and allegations that he helped harass minority voters in the 1950s and 1960s. Biden voted against Rehnquist but did not lead the opposition.

Estelle Rogers, director of the Federation of Women Lawyers, criticized Biden's earlier comments on Bork. "He has already given the Biden seal of approval to a potential Bork nomination. He should probably retract his endorsement," she said.

"I would hope Sen. Biden would take time from his busy schedule to exercise the kind of leadership we expect from the chairman of the Senate Judiciary Committee," Rogers said. "If he can't, he'd be wise to think carefully about resigning his chairmanship."