I think a lot of confusion here comes from people lumping together ultimate and intermediate goals in their definitions of morality. Ultimate goals are parts of your utility function: what you really want. As you said, you can't derive these rationally; they're just there. Intermediate goals, on the other hand, are mental shortcuts, things that you want as a proxy for some deeper desire. An example would be the goal that violent criminals get thrown in jail or otherwise separated from society; the ultimate goal that this serves is our desire to avoid things like being savagely beaten by random ne'er-do-wells when we go to the 7/11 to buy delicious melon bread. But if there were a more effective, or cheaper, or more humane way to prevent violent crime, rationality can help you figure out that you should prefer it.
Rationality can and should define your intermediate goals, but can't define your ultimate goals. But when most people talk about morality, they make no distinction between these. As soon as you do distinguish between these, the question tends to dissolve. Just look at all the flak that Sam Harris is getting for saying that science can answer moral questions. What he's really saying is that science can help us determine our utility functions and figure out how to optimize them. The criticism he gets would probably evaporate if he would taboo "morality" for a little while, but he gets way more media attention by talking this way.
Query: by what objective criteria do we determine whether a political decision is rational?
I propose that the key elements -- necessary but not sufficient -- are (where "you" refers collectively to everyone involved in the decisionmaking process):
If an argument satisfies all of these requirements, it is at least provisionally rational. If it fails any one of them, then it's not rational and needs to be corrected or discarded.
This is not a circular definition (defining "rationality" by referring to "reasonable" things, where "reasonable" depends on people being "rational"); it is more like a recursive algorithm, where large ambiguous problems are split up into smaller and smaller sub-problems until we get to a size where the ambiguity is negligible.
This is not one great moral principle; it is more like a self-modifying working process (subject to rational criticism and therefore improvable over time -- optimization by successive approximation). It is an attempt to apply the processes of science (or at least the same reasoning which arrived at those processes) to political discourse.
So... can we agree on this?
This is a hugely, vastly, mindbogglingly trimmed-down version of what I originally posted. All comments prior to 2010-08-26 20:52 (EDT) refer to that version, which I have reposted here for comparison purposes and for the morbidly curious. (It got voted down to negative 6. Twice.)