Should Rabbis give political sermons?

Dennis Prager (radio talk show host, author, pundit) has written an article, slamming Rabbis for giving political sermons on the High Holy Days. He says that, on these holiest of days, Rabbis should be focussing on spiritual issues, such as personal growth, teshuvah (repentance), and such. To focus on narrow, partisan political issues is wrong:

But those rabbis who do use Rosh Hashanah and Yom Kippur to offer their political views are doing their congregants and Judaism a real disservice.

Rabbis who have used the holiest days of the Jewish calendar to give a sermon on behalf of the Obama health-care bill or to excoriate the Christian right or to expound on any of the many other left-wing positions have cheated their congregants. The primary purpose of the High Holy Days is to have the Jew engage in moral and religious introspection: What kind of person have I been in the past, and what do I need to do in order to be a better person?

He's especially upset because all Rabbis who give political sermons are on the left, and support only leftist causes:

Because separation of pulpit and politics is a conservative value, not a liberal one. Therefore, rabbis with conservative political beliefs do not use their pulpit to advance their political agenda. And because no conservative believes that advancing the conservative political agenda makes you a good person. Like Judaism, we know that becoming a good person demands arduously working on one’s character, not having the right politics.

Now, this latter idea that only Liberal Rabbis give political (and, therefore, liberal) sermons is borderline farsical. It's true that most Rabbis are politically liberal, so most political sermons will lean that way. But, the idea that only non-Orthodox and politically liberal Rabbis (which, he believes, are nearly identical categories) will ever speak politically on the High Holy Days? Come on. NO Rabbis have spoken out against Same Sex Marriage, for example?

Look, I try very hard not to be political in my sermons. I want to make sure that I'm speaking authetnically Jewishly, and even when I believe that my liberal values come from a very Jewish place, I try to err on the side of caution. And, I am extra cautious of partisan politics - I might speak about a cause, but I would be very wary of speaking about a particular bill or program, and I would (almost?) never speak about an individual candidate.

But, would I ever talk politics? Absolutely.

Supporting Same Sex marriage is politics. Supporting caring for the poor is, partially, politics. Opposing slavery was (and is) politcs. Does Judaism have nothing to say, from either side, about health care, or abortion, or war, or...anything which overlaps with politics?

When Isaiah, in a reading which we use on Yom Kippur, said:

Is not this the fast I look for: to unlock the shackles of injustice, to undo the fetters of bondage, to let the oppressed go free, and to break every cruel chain? Is it not to share your bread with the hungry, and to bring the homeless poor into your house?

Is it not possible that there is a least a teensy bit of politics which overlap with that thought?

Look, it is incredilby hard to know where to draw the line - to know when a truly religious issue crosses over into a much more truly political one. It's also incredibly difficult to know when we (who are both Liberal (non-Orthodox) Jews as well as political liberals) hold a belief because it's Jewish, vs. because it's liberal (or, how much of each). When we should be willing to speak our positions, and when we should say, "there are mutliple valid opinions on this, and we have to honor them all." Very, very tricky. Like I said, I err on the side of caution, as I think I should. But, Prager takes a complicated issue, and falsly makes it simple: liberal/politcal=bad. Orthdoxox/Conservative/non-politcal = good.

I'd be curious to hear what you all think - when can a Rabbi (or Priest, etc) talk politically, and when is it wrong? How do you know when a line has been crossed? What would make you walk out of a sermon, if anything?

By the way - during one of my sermons during these High Holy Days, I'll be talking about Israel. Is that ok, Mr. Prager? Or, is it too political?