The death of Justice Antonin Scalia in a presidential election year has set up a challenge for President Obama and his legal advisers. They must weigh the best person to put forward, knowing that Republican senators may block that nominee no matter who is chosen. At the same time, if a Democrat prevails in the presidential election, Mr. Obama’s nominee might be confirmed in the Senate’s lame-duck session or would be an obvious front-runner to be renominated by his successor in 2017. Here are six contenders being discussed by close observers of judicial politics.

Image Judge Merrick B. Garland in Washington in 2008. Credit... Charles Dharapak/Associated Press

Merrick B. Garland

AGE 63.

CURRENT ROLE Judge on the United States Court of Appeals for the District of Columbia Circuit.

BACKGROUND White man. Born in Illinois. Attended Harvard Law School and clerked for Justice William J. Brennan Jr., an Eisenhower appointee. A former federal prosecutor and Department of Justice official with a crucial role in overseeing the investigation into the Oklahoma City bombing. Confirmed 76 to 23, in 1997, with 32 Republicans in favor.