President Bush, right, stands with his nominee for the Supreme Court, John G. Roberts Jr., at the White House
President Bush and his Republican allies in the Senate hope to confirm Judge John Roberts to the Supreme Court by October, when the court's next session begins.  Supreme Court confirmations can be lengthy and politically charged.

The requirement for Senate confirmation of the president's Supreme Court nominees is contained in the U.S. Constitution.

"In the selection of a justice for the Supreme Court, the Constitution actually divides the power between the president and the Senate," said Jonathan Turley, a professor of constitutional law at George Washington University Law School in Washington, D.C.  "Only the president can nominate someone for the Supreme Court.  But only the United States Senate can confirm that individual.  That means that there is some give and take between presidents and the Senate."

In announcing his nomination of Judge John Roberts to replace retiring Supreme Court Justice Sandra Day O'Connor, President Bush urged the Senate to act quickly so that Judge Roberts can take a seat on the high court in time for the next session beginning in October.

"The nation also deserves a dignified process of confirmation in the United States Senate characterized by fair treatment, a fair hearing and a fair vote," said Mr. Bush.

Senate confirmation of a Supreme Court justice requires a simple majority vote.  Republicans control 55 of the 100 Senate seats.  But opposition Democrats are insisting on a comprehensive confirmation process that includes extensive public hearings before the Senate Judiciary Committee.

During the hearings, senators from both parties will question Judge Roberts, including New York Democrat Charles Schumer.

"I want to vote for Judge Roberts," he said.  "But he has to meet the only standard that I have.  Is he an agenda-driven ideologue who will impose his views on the American people or is he a mainstream, albeit conservative, mainstream jurist?"

Confirmation hearings became a customary part of the process beginning in the 1950s.   But in recent years, there has been a debate over whether it is appropriate for nominees to be asked about their views on legal issues or cases that may eventually come before the Supreme Court.  The vast majority of Supreme Court nominees have declined to answer questions about specific cases.

"The thing that the nominee wants to do is not give away too much and to manage to wind up at the end of process without having revealed too much," explained Mary Cheh, a visiting professor at George Washington University.  "And I think Mr. Roberts, having argued 30-some cases before the Supreme Court, having been involved in major litigation, is very adroit at handling questions and answering what he wants to answer."

Perhaps the most contentious confirmation battle took place in 1987 when the Democratically controlled Senate rejected President Ronald Reagan's choice of Judge Robert Bork for the Supreme Court.

Democrats accused Judge Bork of being a conservative ideologue.  Republicans said Judge Bork easily had the legal qualifications to serve on the high court but paid a price for sticking to his conservative views during the hearings.

"There are some senators historically that have given the president whatever nominee he wants unless there is some criminality or something truly untoward in the person's past," added constitutional law expert Jonathan Turley.  "That has changed over the past couple of decades.  The Senate now increasingly asks nominees about their ideology, even how they would vote on specific cases."

Both sides insist they want to avoid the kind of political acrimony that characterized the fight over Judge Bork.  President Bush says he would prefer a repeat of the relatively smooth confirmations for the last two justices, Ruth Bader Ginsburg and Stephen Breyer, who were nominated by President Clinton.

Of the 148 men and women previously nominated to the Supreme Court, 28 (nearly one-fifth) failed to win Senate confirmation.