With the election of Senator Scott Brown, Republican of Massachusetts, Democrats can no longer hold off a Republican filibuster. And while Democrats are emboldened by the health care vote, the passage of the legislation — which is already facing legal challenges from Republicans who say it is unconstitutional — has left the Senate more polarized than ever and created a climate in which the courts could easily become an election issue.

For the court, Justice Stevens’s departure will be the end of an era. He is the longest-serving justice by more than a decade, and he is the last remaining justice to have served in World War II. (He joined the Navy, where he served as a cryptographer, the day before Pearl Harbor was attacked.) His leaving will not, however, change the composition of the court; although he was appointed in 1975 by President Gerald R. Ford, a Republican, he has become one of its most reliably liberal members during his nearly 35-year tenure, as the court drifted ever rightward.

Still, for Mr. Obama, who taught constitutional law at the University of Chicago (where he was a colleague of Judge Wood), the vacancy is an unmistakable chance to put his stamp on the direction the court takes for the next several decades. Mr. Obama is already engaged in an unusual public confrontation with the court over its recent decision in the Citizens United case, which lifted strict limits on corporate spending in elections. On Friday, during a brief appearance in the Rose Garden, he made clear that the case was very much on his mind.

He vowed to “move quickly” in announcing a nominee. Senior advisers said they expected a decision within the next several weeks. The president said he would look for a candidate who possessed what he described as qualities similar to that of Justice Stevens: “an independent mind, a record of excellence and integrity, a fierce dedication to the rule of law and a keen understanding of how the law affects the daily lives of the American people.”

And, in what legal scholars took as a clear swipe at the Citizens United decision (for which Justice Stevens wrote the dissent), the president said he would look for a justice who “knows that in a democracy, powerful interests must not be allowed to drown out the voices of ordinary citizens.”

The White House already has a Supreme Court nomination team in place, with the selection process run by the new White House counsel, Robert F. Bauer, and overseen by Rahm Emanuel, the chief of staff. Once a nominee is picked, Mr. Bauer’s wife, Anita Dunn, who is Mr. Obama’s former communications director, will coordinate with advocacy groups. Vice President Joseph R. Biden Jr., who was chairman of the Senate Judiciary Committee during some of its most contentious confirmation fights, is also likely to play a crucial role.