Supreme Court Justice Byron R. White is emerging as a critical fifth vote in the increasingly rare instances in which the four liberals on the court prevail in 5 to 4 decisions.

Last week, White provided the fifth vote when the court effectively outlawed political patronage by ruling that party affiliation could not be used as a basis for hiring, promoting or transferring government employees.

So far this term, White has been the deciding vote in six of the eight 5 to 4 victories, including the patronage ruling, for the court's liberal faction -- Justices William J. Brennan Jr., Thurgood Marshall, Harry A. Blackmun and John Paul Stevens.

Conversely, however, White was the fifth vote in 11 of the 13 rulings in which the conservative group -- Chief Justice William H. Rehnquist and Justices Sandra Day O'Connor, Antonin Scalia and Anthony M. Kennedy -- prevailed.

The 73-year-old White has voted in the majority this term more often than any other justice.

White's role at the center appears to stem from changes in the court's membership rather than any shift in his judicial philosophy after 28 years on the court.

"I don't think it's that he's changed," said University of Chicago law professor Geoffrey P. Miller, who clerked for White during the 1979 term. "I think it's that Scalia and Kennedy and O'Connor have moved the court . . . so that White is now the centrist vote."

Of the 49 rulings this term in which the justices split 5 to 4 or 6 to 3, White voted with the majority in 43, compared to 36 for Kennedy, 35 for Rehnquist, 34 for O'Connor and 32 for Scalia.

Of the 26 decisions in which the court voted 5 to 4, White was in the majority all but five times. He dissented from the decision striking down the federal flag-burning law and in four relatively minor cases.

Until his retirement in 1987, Justice Lewis F. Powell Jr. served as the court's swing vote. After Powell was replaced by Kennedy in 1988, many court observers expected O'Connor to hold the balance of power. That expectation has proved accurate in some of the most controversial areas under the court's jurisdiction -- abortion, affirmative action and church-state conflicts -- and could be borne out again at the end of this term, when the court is expected to rule in two abortion and two affirmative action cases.

But O'Connor has appeared to grow more conservative in other areas, and Kennedy has turned out to be more consistently conservative than many predicted at the time of his nomination.

Last year, the four liberals picked up O'Connor's vote in three of the five cases in which they held a 5 to 4 majority, and snared White as the fifth vote just once, in a death penalty case. This year, O'Connor has not once sided with the liberal faction in a 5 to 4 decision.

White, consequently, holds the key to decisions in areas other than abortion, affirmative action and religion, particularly cases involving criminal law and the role of the federal courts.

"It depends on the kind of case which one of them {White or O'Connor} is going to be the swing vote," said Duke University law professor Walter Dellinger, who secured White's vote along with the four liberals in a decision this year giving hospitals the right to sue in federal court for reasonable Medicaid reimbursement rates. "In an area where Justice O'Connor is conservative, Justice White is the only possible fifth vote" for the liberals, he said.

Two pairs of 5 to 4 cases at the high court this year illustrate White's central role.

One set of decisions involved the power of federal judges to impose remedies in civil rights cases. In one, Spallone v. U.S., White voted with the conservatives to overturn fines imposed by a New York judge against members of the Yonkers City Council for refusing to implement a housing desegregation plan.

In the other, White joined the liberals to uphold the power of a federal judge in Kansas City to order the local school board to increase property taxes to pay for a sweeping school desegregation plan.

In two criminal cases involving the use of illegally obtained statements for purposes of impeaching -- or casting doubt on -- testimony, White sided once with conservatives and once with the liberals.

The case in which the conservatives won, Michigan v. Harvey, involved a Michigan man whose statements to police were used to impeach his testimony that he did not rape a woman. Although the statements were taken in violation of the man's constitutional right to counsel, the majority said, they could be used to rebut his testimony.

In the other case, James v. Illinois, Darryl James, arrested in a shooting by a gunman with "reddish" hair, told police that he had just changed his hair color from reddish-brown to black. The statement could not be used against James directly because police lacked probable cause when they arrested him without a warrant.

But a friend of James's testified at trial that James had black hair on the day of the shooting. The state sought to use James's statement to impeach his friend's testimony, arguing that refusing to allow the statement for those purposes would permit defendants to commit "perjury by proxy."

White joined with the liberals in that case to vote to suppress the statement, averting what criminal defense lawyers said would have been a major erosion of the exclusionary rule, which prohibits the use of illegally obtained evidence.

"In at least some of the situations where some of his colleagues are being judicially activist in their conservativism he won't be with them, but where the conservative position is a judicial restraint position he's more likely to be with them," said Harvard Law School professor Lance Liebman, who clerked for White during the 1967 term.

Liebman noted that White has found himself in the center position before, during the early 1970s when the court was split between four justices appointed by President Nixon -- Rehnquist, Powell, retired Chief Justice Warren E. Burger and Blackmun, who then was far more conservative -- and four Warren court holdovers -- Brennan, Marshall, William O. Douglas and Potter Stewart.

"He's been doing this a long time, and those alignments keep changing and I bet you he just smiles about" the cases in which he is the critical vote, said Liebman, who wrote a 1972 New York Times Magazine article pointing out White's role as the "swing man" on the court. "If you're there a long time, you know these things come and go."