I have written this piece (free access, but registration required) for The New Republic Online. It begins:

As Chief Justice of the Supreme Court, William Rehnquist is used to wielding a great deal of power. But now Rehnquist, who has been battling thyroid cancer, is in an unparalleled position of power: He may be the only person who can prevent the Senate from losing its character as a deliberative institution and devolving into a second House of Representatives. And he can do so by making a decision to retire now from the Supreme Court.

It ends:

To be sure, it might seem beneath the dignity of the Chief Justice to base his retirement decision on these kinds of political matters. But justices almost certainly take partisan politics–namely, the ideological leanings of the president who would be appointing their successor–into account when making decisions about when to step down. The difference here is that instead of seeking to ensure his replacement with a like-minded conservative, Rehnquist would be seeking to protect the very process by which Supreme Court justices are confirmed.
Indeed, if Rehnquist passes up this opportunity to retire, things might only get worse. A retirement at the end of the next term would come just months before a congressional election, and fallout from the O’Connor replacement process–not to mention election-year politics–would inevitably cloud the confirmation. By 2007, we will already be entering a new presidential election season and the choice of Rehnquist’s successor would be made by a president who already has one foot out the door. These are not palatable options for a sharply divided nation; all raise the likelihood of further dividing the Senate. Fortunately, one man can forestall these scenarios. But he needs to act now.