Why the Hypocrisy Defense is political suicide for liberalism

All across the internet over the last few hours, liberal commenters and bloggers have fallen back on one of their most trusted logical arguments in situations like this in which a Democrat is caught in a sex scandal: “At least he’s not a hypocrite.”
This sampling of (unedited) comments taken from today’s New York Times‘ and San Francisco Chronicle‘s articles about Anthony Weiner’s public confession are typical:

“Much to do about nothing. Please wake me up if you find out that he mishandled/stole taxpayer money, or had previously participated in some sort of moral clensing crusade.”

…and…

“Unless an elected official is a hypocrite (i.e., an anti-gay politician who espouses “family values” but solicits men for sex in public restrooms), I don’t care about his private life, including whether he sends naked pictures of himself to women who aren’t his wife.”

…and…

“I certainly find it reprehensible, particularly the lying. But he didn’t run on a Family Values moral superiority platform, like Ensign; there is less hypocracy and more simple stupidity here.”

A quick search of the liberal blogosphere and in the comments sections of MSM articles will turn up countless similar examples. If you spend any time on the Internet, you’ve undoubtedly encountered it yourself over and over, as others have noted. And it hasn’t just emerged in regards to Weinergate: It’s actually one of the bedrocks of the liberal worldview: Conservatives are hypocrites concerning moral issues, whereas liberals are not.
Which got me to thinking:
This has to be the weakest philosophical argument I’ve ever encountered.
Not just weak: self-extirpating.
If there ever was a moment to really dig down into the fundamental structure of this argument, this is it. So let’s get down to brass tacks, shall we?
Sleight-of-mind
What liberals really really love about this stance is its climactic declaration: Our opponents are hypocrites!
Here is how the liberals present their case:

But what they don’t want you to think about — and what they themselves don’t even want to acknowledge — is that this “hypocrites” howl is the second half of a two-part argument. And in that second half, they are the victors. But in the first half….
Well, for the “at least we’re not hypocrites” sentiment to make sense, there must be an agreed-upon starting point — one which the liberals themselves are confirming each time they make this argument. And what must that starting point necessarily be? For conservatives to be hypocrites when they do something immoral, then that means they must profess a moral ideology in the first place. And — here’s the key — for the liberals to be let off the hook when they do something immoral, then that means they must profess an ideology with no moral claims whatsoever.
Thus, the diagram above only showed you the climactic second half of the liberals’ sleight-of-mind trick. The full statement — including the first half which you’re not supposed to think about — would be diagrammed like this:

Not quite so effective an argument when seen this way, is it?