WASHINGTON — The most important number in President Barack Obama's consideration of a Supreme Court nominee may be 50.
That's the age of Solicitor General Elena Kagan, who hit the half-century mark in late April. If Obama nominates Kagan to the high court, her age could be the decisive factor.
Kagan is the youngest, by nearly seven years, of the four people the president is known to have interviewed to succeed retiring Justice John Paul Stevens. She would be the youngest nominee by a Democratic president since 1962, when President John F. Kennedy chose 44-year-old Byron White.
Age is one of many considerations before Obama as he closes in on filling the vacancy that will be created by Stevens' retirement this summer. The president could announce his pick any day.
One reason that age could be important is that this appointment is unlikely to immediately affect the court's balance of power. There was a shift when President George W. Bush chose appeals court Judge Samuel Alito to replace Justice Sandra Day O'Connor, a moderate whose vote often made the difference in ideologically charged cases. Alito is now considered to be one of the more conservative justices.
This time around, Obama is certain to name a left-of-center successor to Stevens, the court's leading liberal. Finding a younger justice, who theoretically would serve longer, could enhance Obama's legacy.
Picking younger justices has been a hallmark of recent Republican presidents. The current chief justice, John Roberts, was 50 when Bush nominated him, and Clarence Thomas, chosen by Bush's father, was 43. Other than Alito, who was 55, every GOP-nominated justice back to O'Connor was 50 or 51.
The White House has given every indication that it will follow the same strategy that worked last year when appeals court Judge Sonia Sotomayor easily won confirmation. Rather than nominate someone largely because of an ability to make it through the Senate, Obama chose someone he wanted. Sotomayor's powerful personal story carried the day.
Obama has mainly avoided choosing liberal stalwarts for the bench so far. He may again avoid a major confirmation fight by opting for a candidate with a track record of moderation and a reputation for working well with conservatives.
Or he could go for broke in a Senate where Democrats control 59 votes: a nominee with a clear liberal voice, who might elate Obama's political base, but force him to use some political muscle to win confirmation.
"Based on what he's done so far, it doesn't seem like he is willing to expend a lot of political capital," said Christine Nemacheck, a government professor at the College of William and Mary in Williamsburg, Va. Nemacheck is the author of "Strategic Selection," a book on how presidents choose high court nominees.
With polls showing a drop in Obama's popularity and the prospect for Republican gains in November's elections, Nemacheck said that if the president wants to put a strong liberal on the bench, "Now probably is his best chance."
Obama could further diversify the court by giving it another woman to join Justices Ruth Bader Ginsburg and Sonia Sotomayor, his first nominee a year ago. He may settle on a nominee who is Jewish or Catholic and leave the court with no Protestant justices for the first time in history. If regional balance is important, he could choose someone who is not from Boston or Washington, or points in between.
Obama began with about 10 candidates but appears to be centering on the two women and two men known to have had interviews with him: Kagan and three federal appeals court judges — Diane Wood, 59; Merrick Garland, 57; and Sidney Thomas, 56.
None of the four seems likely to provoke an all-out fight over confirmation, although conservative interest groups already have expressed varying degrees of opposition to each.
Carrie Severino, chief counsel of the Judicial Crisis Network, a group that says it's committed to the Constitution and the Founding Father's "vision of a nation of limited government," said Obama might pick Kagan because she has a relatively thin paper trail for critics to pick apart. Yet Obama could be sure that, as a justice, Kagan would "rubber stamp an agenda he knows the American people are opposed to," Severino said.
The top contenders also have in common praise from conservative colleagues for their ability to bridge partisan divides.
In Kagan, Obama would have a young justice who has not been a federal judge. She would be the only person on the court who had not previously been a judge.
Garland is the least likely to excite Republican opposition and is personally friendly with Roberts, his former colleague on the federal appeals court in Washington.
Wood, a Chicagoan with a University of Texas law degree, and Thomas, born and educated in Montana, would bring regional diversity. Either would be the only justice without an Ivy League education.
Wood and Thomas are Protestants. Garland and Kagan are Jewish.