The U.S. Senate has begun debate on the nomination of Judge John Roberts to be the next Chief Justice of the United States. But with his confirmation virtually assured by the Republican-led Senate, lawmakers are turning their attention to the second vacancy on the high court.

Senate Republican Leader Bill Frist opened debate by praising the Chief Justice nominee. "John Roberts embodies the word that should be synonymous with every judge: he is fair," the senator said.

But some Democrats are concerned by Judge Roberts' record on abortion and civil rights when he worked as a lawyer in the Reagan administration. Senate Democrat leader Harry Reid has already announced his opposition to the nomination. "There are strong feelings on each side," he said.

The Senate is expected to vote later this week on the nomination, just days ahead of the start of the new Supreme Court session next Monday. With Republicans holding 55 of the 100 seats in the chamber, there is little question that Judge Roberts will be confirmed.

So many lawmakers are focusing on the second vacancy on the Supreme Court.

President Bush is considering candidates to succeed retiring Justice Sandra Day O'Connor, and could announce his decision as early as this week.

Justice O'Connor has often cast the deciding vote in close five-to-four rulings in a number of controversial cases.

For that reason, Democrats are concerned Mr. Bush may nominate a conservative to the bench to appeal to his conservative political base, thus tilting the high court to the right.

The top Democrat on the Judiciary Committee, Senator Patrick Leahy, called on the President to name a moderate to succeed Justice O'Connor:

"We are asking him, in this case especially, be a uniter, do not be a divider, for the sake of the country, not for the sake of 100 senators, but for the sake of the country," he said.

Speaking to reporters, Mr. Bush raised the possibility he would nominate a woman or a minority to the Supreme Court, which is dominated by white men. "I will put a person in to do the job. But I am mindful that diversity is one of the strengths of the country," he said.

But the chairman of the Judiciary Committee, Republican Senator Arlen Specter, cautioned that race or gender should not be the overriding factor in choosing the next Supreme Court Justice.

"There is no doubt that Justice O'Connor was a swing vote, tipping the scale. I believe that is a factor to be considered. While I would like to see more women on the court, and I would like to see a Hispanic on the court, and I would like to see more African Americans on the court, I urge the President to name the very best person he can find," he said.

Whomever Mr. Bush nominates, Senator Specter says he expects a contentious hearing, since the court's balance of power is at stake. Justice O'Connor has said she would remain on the bench until her successor is confirmed.