WASHINGTON – The U.S. Senate opened a spirited debate on U.S. Supreme Court nominee Sonia Sotomayor -- with history indicating it is impossible to know how she may ultimately rule on the highest U.S. court.

With Democrats in control of the Senate, President Barack Obama's nomination of the first Hispanic to the Supreme Court was expected to be confirmed by week's end. But if past is precedent, it may take years to determine what sort of justice she will be.

Liberal and moderate justices have become conservatives and vice versa -- at least on some key issues like civil rights, abortion and gay rights.

There's no way you can tell, Senate Judiciary Committee Chairman Patrick Leahy, a Democrat and Sotomayor backer, said in a recent interview. Judges can change during their lifetime appointments.

Sotomayor, 55, a federal judge for the past 17 years, is viewed by backers as a moderate who rules by the law but by critics as a liberal who allows personal feelings to influence her decisions.

Senator Jeff Session, top Republican on the Judiciary committee, ripped into Sotomayor in a Senate speech.

But he admitted: I am unable to predict with certainty how Judge Sotomayor will perform if confirmed ... History shows that justices, once confirmed, often surprise.

Consider former Chief Justice Earl Warren, appointed in 1953 by Republican President Dwight Eisenhower. Warren became a liberal lion on the court and Eisenhower called the appointment the biggest damned-fool mistake I ever made.

President John Kennedy never made such pronouncements, at least publicly, about Byron White, who he appointed to the court in 1962. But many Kennedy backers were disappointed that White was less liberal and more conservative than expected.

More recently, David Souter, who Sotomayor would replace, was seen as a conservative when the first President George Bush appointed him to the court in 1990.

But Souter ended up on the liberal wing of what has been a sharply divided court, one that in recent years has routinely ruled 5-4 in favor of conservatives.

Republican Senator Lindsey Graham announced last month he would vote to confirm Sotomayor, but voiced some uncertainty.

'TIME WILL TELL'

I think that Judge Sotomayor will not be any more liberal than him (Souter), and on some issues, quite frankly, may be more balanced, Graham said. Time will tell.

Harry Blackmun, a 1970 appointee of President Richard Nixon, was expected to be a conservative justice. Yet he wrote the 1973 Supreme Court ruling that legalized abortion and still riles Republicans. He also reversed himself and opposed the death penalty.

I assume Nixon regretted the pick, said Jamal Greene, a professor at Columbia Law School who clerked in 2006 and 2007 for Supreme Court Justice John Paul Stevens.

Stevens was appointed in 1975 by Republican President Gerald Ford. He was then seen as a moderate but emerged as a liberal on the high court.

He says he hasn't changed, but that the court moved to the right on him, said Greene.

Current justices on the nine-member court have for the most part performed as expected as Senate scrutiny has become tougher over the last 50 years, Greene said.

He said Souter had been the biggest surprise by far in recent years, adding that Anthony Kennedy, another Republican appointee, had been more liberal than expected on at least some issues, including gay rights and the death penalty.

Republican Leader Mitch McConnell and Senate Democratic Leader Harry Reid raised the curtain on the Sotomayor debate.

McConnell said he opposed Sotomayor largely because of what he said was Obama's focus on empathy in looking for a justice.

Empathy is only good if you're lucky enough to be the person or the group that the judge in question has empathy for, McConnell said.

Reid brushed off such talk and made a prediction about the first Hispanic Supreme Court nominee. This week the Senate will make history when we confirm her.

(Reporting by Thomas Ferraro; editing by David Alexander and Todd Eastham)

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