Elsenet today I was talking with a fellow who asked me this. While the question was clearly a bumpersticker more than a serious question, I have a passion for answering rhetorical questions, and so (with a couple of fixes) here is my reply. It's not suave or carefully worded, and in fact a couple spots still make me wince, but perhaps its informality has a certain power too. Enjoy.
So, "protected classes." Once upon a time, our Founding Fathers authored a Declaration of Independence, which contained a document that set as a philoosophical foundation the idea that "all men were created equal", and that natural rights emanated from that equality. Of course, in my view, some of the implementation, as the country went on, proved a little short-sighted; a fact which lead to the bloodiest war in American history, and as a result, the fourteenth amendment.
The fourteenth amendment nods to a fact that it's fairly easy for a large population to decide that a minority population is "less than" and to create laws which target them specifically. This is borne out by history, in matters great and small, from the worst (Jews, gypsies, gays, etc., in Nazi Germany) to smaller-than-genocide but still serious and important matters (literacy tests which were only applied to Black voters.)
One challenge in applying the fourteenth amendment is determining what does and doesn't count. All laws distinguish one group from another. Obviously "thief" is not a group we worry about targeting unfairly. At least, I don't. So what groups do we worry about?
There is a necessary balance between the power of the legislature to get work done, and the underlying "treat people equally" Constitutional requirement of the 14th (and, we argue, the 5th) amendments. In the vast vast majority of cases, the legislature merely requires a "rational basis", that is, a plausible excuse, to pass any law they please. This is very sensible, and pragmatic.
However, in a limited number of cases, the Founding Fathers, or the government, or the courts have found, that particular groups get fucked by this. With respect to religion, for example, the problem was so large that a note was made in the very first amendment which provides some protections for that class, and the 14th amendment fairly directly adds race to that list. It was the US. vs. Caroline Products decision, Footnote Four, which really started laying out the predicates as to when "rational basis" isn't enough oversight. Those predicates are:
1. Have they been fucked over for a long time?
2. Do they have the power to stop themselves from being fucked over in the government?
3. Did they have a choice about why they're being fucked over (e.g., the "thief" case), or not (skin color) If not, that's an indicator too.
Groups that meet these tests (or, which, as in the case of race and religion are nodded to in Constitutional text) are considered protected classes. That's all they are. And they're power is that they permit a somewhat greater judicial oversight of legislative intent in lawmaking, to address the question of "Are they being fucked over?"
Liberals like this because we believe that powerless groups should not be inherently fucked over, and that some level of pragmatism (e.g., not having every GD'd law spend twenty years in courts, just double-check the few that actually address people who really truly are being fucked over) is sensible.
Hope this helps. Love, me.