Justice Souter has announced his imminent retirement:

Justice David H. Souter has indicated that he plans to retire at the end of the term in June, giving President Obama his first appointment to the Supreme Court, three people informed about the decision said Thursday night.

Justice Souter, who was appointed by a Republican president, George H. W. Bush, but became one of the most reliable members of the court’s liberal wing, has grown increasingly sour on Washington and intends to return to his home state, New Hampshire, according to the people briefed on his plans. His decision was first reported by National Public Radio.

The decision opens the first seat for a Democratic president to fill in 15 years and could prove a test of Mr. Obama’s plans for reshaping the nation’s judiciary. Confirmation battles for the Supreme Court in recent years have proved to be intensely partisan and divisive moments in Washington, but Mr. Obama has more leeway than his predecessors because his party holds such a strong majority in the Senate.

The early consensus seems to be that Justice Souter’s retirement will not have any material impact on the court:

Replacing Justice Souter with a liberal would not change the basic breakdown on the court, where he and three other justices hold down the left wing against a conservative caucus of four justices. Justice Anthony Kennedy, a moderate Republican appointee, often provides the swing vote that controls important decisions.

However, that depends on which President Obama steps up to nominate his successor. If it turns out to be the campfire-Kumbaya Obama, still desperate to cement his bipartisan credibility, we could well end up with a moderate appointee, shifting the center of gravity of the court in ways that don’t bear thinking about.

And when I think about some of the things Obama has cheerfully done — his stance on warrantless wiretapping, his wholehearted embrace of the state secrets privilege, his steadfast refusal to put anyone in the dock for the decision to make us state practitioners of torture — I’m not so sure that Obama will choose to nominate a justice as liberal as Souter turned out to be.

The Democratic caucus may have 59 votes but, well before Souter’s announcement, the Republicans had already been preparing to go berserk whenever Obama’s first Supreme Court nomination came up. Jon Kyl threatened to filibuster U.S. Supreme Court appointments back in November:

Jon Kyl, the second-ranking Republican in the U.S. Senate, warned president-elect Barack Obama that he would filibuster U.S. Supreme Court appointments if those nominees were too liberal.

Nobody doubts that Republicans will be jumping up and down in fits of apoplexy if Obama picks a reliably liberal nominee. Obama has in the past shown himself to be hypersensitive to such anticipated Republican rancor, and has acted in the flawed belief that meeting Republicans halfway will serve to mitigate the rancor. Obama could very well choose to reprise that behavior again.