I would certainly agree that it is highly improper for a religious leader to endorse or oppose particular candidates. Even when such opinions are expressed as personal opinions there remains potential for a perception of an institutional endorsement. What's more, taking such specific public positions risks linking the religious institution to one or the other party or candidate, and thus potentially alienating members of its own flock who might support the other side.
However, it is irrational to suppose that religious institutions and religious leaders should be silent regarding all things political. Indeed, it is impossible. While it would be immoral (if not illegal) for a religious institution to endorse particular candidates or parties, it would be hypocritical for a religion to proclaim certain values and then remain silent in the face of political issues that directly relate to those values. This is decidedly different from supporting or opposing candidates and political parties. To argue otherwise is ipso facto to deny to religion the right to a voice in the public sphere and the right to integrity in what it proclaims.
There's a lot more on this subject over on his blog, so I encourage all of you to go check it out. If you've got something to say, I encourage you to take a minute to go over to his blog and say it.