We talked this morning about the Democrats’ poor electoral position — already shaky, it is probably now deteriorating further — but we haven’t talked as much about why they are in this predicament. This is for a good reason: once you get past the premise that the state of the economy plays a large role (something that pretty much everyone would agree with), this is a difficult question to answer.

The reasons for the Democrats’ decline are, as we say in the business, overdetermined. That is, there are no lack of hypotheses to explain it: lots of causes for this one effect. The economy? Sure. Unpopular legislation like health care? Yep. Some “bad luck” events like the Gulf Oil spill? Mmm-hmm. The new energy breathed into conservatives by the Tea Party movement? Uh-huh.

And this hardly exhausts the theories. An inexperienced White House that has sometimes been surprisingly inept at coping with the 24/7 news media cycle? The poor optics associated with Democrats having had a filibuster-proof majority in theory, but not always in practice? All of the above.

These causes can’t be so easily untangled on the basis of polling evidence; there’s really no basis on which to evaluate the competing hypotheses. This is particularly so given that different types of political events aren’t isolated from one another — health care reform might have been unpopular, for instance, but the reason for its unpopularity may ultimately have been the economy.

For this reason, we can be skeptical of two types of analysis: claiming that Factor X definitely isn’t contributing to the Democrats’ troubles, and asserting that it definitely is. For instance, I’d urge some caution in reading this article at Real Clear Politics by Jay Cost — which rightly critiques those who have entirely dismissed the role that health care played in the Democrats’ decline, but probably goes too far in arguing the contrary. Mr. Cost is right, for instance, that the Democrats’ decline in the polls was steepest last summer, when the health care debate began — but when one delves in a little deeper, the timing of the sharpest periods of decline do not line up well with specific events in the health care debate.

Does that mean Mr. Cost is wrong? Not at all. Health care dominated the political discourse for about nine months; it seems implausible that it hasn’t played some role. But he hasn’t offered much in the way of proof — nor is there much of it to be had: overdetermined phenomena usually beget underdetermined attempts to explain them.