Justice Kagan is going to need to recuse herself from quite a few cases this term because, as Solicitor General, she was involved in the government's preparation for the case. This is as it should be. But Senator Leahy doesn't want to give up on one of the liberal votes. So he's come up with another idea - how about letting retired justices sit in on the case in place of the recused justice.
It strikes me that this is unconstitutional. The Constitution establishes how justices will get on to the Supreme Court. The president nominates and the Senate approves. It says nothing about bringing in pinch hitters from people who have already retired. While Congress had the power to establish the court system and determine the number of justices on the Court, this seems to be a different matter since it would establish a path to the Court other than the one laid out in the Constitution.
Of course, Leahy isn't concerned about the Constitution. He is just looking for a way to replace one justice with one of the retired justices. As the WSJ points out, that would be either John Paul Stevens, David Souter, and Sandra Day O'Connor. Stevens and Souter are reliably liberal and O'Connor goes back and forth, but in her later years often voted with the liberals. How convenient for Leahy. It's just too bad if he is clearly getting around the Constitution.
This bad idea will die without getting through the Senate, but it does allow Leahy one more chance to trumpet the Democrats' message that they think the Supreme Court is too conservative and partisan. That's their accusation whenever it comes up with a decision they don't approve of. Of course, if they make a decision that the liberals like, then partisanship is no longer a problem.