The upcoming battle over President Reagan's eventual nominee to the Supreme Court is likely to be as much a debate about the Senate's proper role in the confirmation process as about the qualifications of the potential associate justice.

In recent years, those opposing nominations have based their arguments on perceived character defects, misconduct or other personal flaws rather than engaging in an all-out assault on the nominee's political or judicial philosophy.

This time around, things are likely to be different. Senate Democrats have signaled their intention to take on any nominee who is, in the words of Majority Whip Alan Cranston (D-Calif.), an "ideological extremist." Cranston called on his colleagues Monday to form a "solid phalanx" to block an "ideological court coup" by Reagan.

Most significantly, Sen. Joseph R. Biden Jr. (D-Del.), chairman of the Senate Judiciary Committee, who last year voted to confirm Justice Antonin Scalia, seems to be taking a different, more aggressive approach to the vacancy created by the retirement of Justice Lewis F. Powell Jr., a pivotal vote on some of the most controversial issues before the high court.

In a statement issued the day Powell announced his retirement, Biden said his position could turn on whether the president's choice "would alter the balance of the court."

Conservatives contend that, if the Senate were to reject a nominee solely on ideological grounds, it would be straying far beyond its constitutional prerogative. The president has the power, they argue, to place his nominees on the court unless they are intellectually or morally unqualified.

"There's no question that the Senate has every right to review a candidate . . . but there has to be a line drawn when they find that the individual is qualified and he's the choice of the president and there's no major problem with his character or his qualification or his background," said Daniel J. Popeo of the conservative Washington Legal Foundation.

"I don't think it's responsible . . . if they're found qualified, that the ideological disagreement with them is the sole basis for saying that they should not sit on the bench," he said.

"What do they want to do, clone Lewis Powell? That's silly," said Patrick McGuigan of the Free Congress Foundation. "It will be up to liberals to explain what the justification is for opposing a qualified man or woman who seeks merely to apply the law."

Liberals argue that the Constitution gives the Senate a mandate to provide "advice and consent" to the president on nominations that includes consideration of the candidate's ideology or any other relevant factor.

"The president took an oath to defend the Constitution. That means he has an obligation to pick people who share his vision of the Constitution," said American University law Prof. Herman Schwartz. "Senators took the same oath," he said, and each "has an obligation to make sure nobody would destroy the Constitution as that senator sees it."

Schwartz said that "the Constitution gives the president the right to try" to appoint justices in tune with his own philosophy. But, he said, "It doesn't give the president the right to succeed. Otherwise, it wouldn't have given the Senate half the power."

The Senate has rejected almost one of every five presidential nominees to the high court, according to a study by Harvard Law School Prof. Laurence H. Tribe, a liberal now advising Biden. The first was Justice John Rutledge, George Washington's candidate for chief justice in 1795, defeated by his fellow Federalists because he opposed a treaty with Britain.

The number might have been higher, said University of Chicago law Prof. Philip Kurland, except that, in the past, "the Judiciary Committee would be consulted before the nomination went to the committee."

Kurland, a moderate conservative who serves as an unofficial adviser to Biden, said "it is not any more unfair for the Senate to have ideological grounds to oppose a nominee than for the president to nominate someone on those grounds."

Legal scholars on both sides of the issue agree that the record on the propriety of voting on the basis of nominees' ideology is ambiguous, in large part because nominees are not often defeated and because ideology has not generally been a major consideration.

"The modern view," said University of Michigan Law School Prof. Yale Kamisar, a liberal, "is that a president should get his pick once you satisfy considerations of ability, integrity and competence. People only recently have made a contrary argument. But it never came to a test before because presidents really never gave that much weight to ideology. With all respect, a Supreme Court appointment was not considered that important until 20 years ago."

But scholars also agree that candidates have been rejected or stalled largely on political grounds -- even when their confirmation would not have altered any ideological balance on the court.

Examples, in addition to Rutledge, include the nomination of Justice Louis D. Brandeis, delayed for almost a year because the American Bar Association and business interests opposed him because he was too liberal -- and, some believe, because he was Jewish.

The Senate rejected John Parker, a federal judge from North Carolina, for the high court in March 1930 by 41 to 39. According to Tribe, Parker was rejected, not because he was unqualified, but because of an anti-labor ruling and anti-black statements.

In 1968, President Lyndon B. Johnson's nomination to elevate Justice Abe Fortas to succeed Earl Warren as chief justice was withdrawn after a filibuster by Sen. Strom Thurmond (R-S.C.), now the ranking minority member of the Judiciary Committee.

Thurmond attacked Fortas for high court decisions on such issues as crime, obscenity, state-federal relations and subversive activities.

Fortas, Thurmond said, "has shown, by his record on the Supreme Court, that he is not only content with the court's trend in these cases, but also that he is willing to take these trends to further extremes. By refusing to confirm this nomination, we can reassure a concerned America that standards do exist, and that action will be taken to preserve them."

Then-Senate Minority Leader Everett M. Dirksen (R-Ill.) reversed his vote for cloture to end the filibuster on the Fortas nomination because he disagreed with the court's decision in an Illinois death penalty case.

Fortas subsequently returned to private practice.

Earlier, 19 Republican senators -- including Thurmond and Howard H. Baker Jr., now White House chief of staff -- signed a letter expressing their "strongly held" view that, given the presidential election four months away, the vacancy should be filled by the new president.

"We will therefore . . . vote against confirming any Supreme Court nominations of the incumbent president," they said.