And away we go.  The AP reports today that Justice John Paul Stevens, the longest-serving and oldest member of the Supreme Court, will retire at the end of this session.  His departure provides Barack Obama an opportunity to make his second pick for the court, but it won’t have any immediate impact on the court’s ideological balance:

Supreme Court Justice John Paul Stevens, the court’s oldest member and leader of its liberal bloc, says he is retiring. President Barack Obama now has his second high court opening to fill.

Stevens says he will step down when the court finishes its work for the summer in late June or early July. …

The timing of his announcement leaves ample time for the White House to settle on a successor and Senate Democrats, who control 59 votes, to conduct confirmation hearings and a vote. Republicans have not ruled out an attempt to delay confirmation.

The leading candidates to replace Stevens are Solicitor General Elena Kagan, 49, and federal appellate Judges Merrick Garland, 57, and Diane Wood, 59.

This will give Democrats an even bigger headache heading into the midterms, especially in the Senate. They just passed a massive government overhaul of health care that was deeply unpopular with voters. If Obama picks a radical jurist along the lines of a Dawn Johnsen or Harold Koh, Republicans will use that against every Democratic Senate incumbent in a year that already looks bad for them.

They may have been better off if Stevens waited another year to retire — although that may have presented its own problems. Right now, the GOP could filibuster any nomination, although that seems pretty unlikely, given the nature of the appointment. Next year, Obama may not be able to get anyone through on an up-or-down vote if the GOP picks up enough seats in the midterms. Stevens gave plenty of hints over the last year that he was going to hang up his robes (he only hired one clerk, traditionally the sign of impending retirement), some of which came before Democrats’ polling crashed through the floor, so he’s probably not concerned about the timing. Democrats should be.

I’d expect to see a nominee with a sparse track record, in order to avoid the kind of scrutiny Goodwin Liu is currently receiving, and someone younger rather than older. Elana Kagan fits the bill, especially since Obama would like to appoint another woman to the court.

Update (AP): Like Ed says, the timing was surely driven by the likelihood of major Republican gains in the Senate next year. By quitting now, Stevens leaves Obama free to appoint a more liberal judge than would be possible in 2011 and don’t think the left won’t use that as leverage to pressure the White House to name a hardcore liberal to the seat. Frankly, given the near certainty already of a red wave in November, The One has little to lose by taking their advice. Kagan isn’t radically liberal as far as I know and might even make it through a closely divided Senate, so don’t be surprised if he passes her over this time and keeps her in reserve in case Ginsburg steps down in the next two years. This may be his last chance to go for broke with a radical. Why not try it?