In the many discussions of the Democratic nomination on this page - we have talked about the fact that Hillary Clinton and Barack Obama will have to make arguments (or, as Bob Shrum has said, moral claims) for the nomination. Such claims will have two general points - legitimacy and electability. That is, candidates will argue why they are the true choice of Democrats and why they are more likely to prevail in November.

We have not heard a lot of squawking from Clinton and Obama about legitimacy just yet. Part of this has to do with the fact that, by any reasonable metric, Clinton is still in second place. Be it pledged delegates or the vote count - Obama has at least a slight lead. This obviously precludes her from making a strong claim to legitimacy just yet. It is sensible for her not to discuss legitimacy much at all. She does not want to set some standard that she ultimately does not meet.

So, most of the back-and-forth between Clinton and Obama's flacks has involved arguments for electability. I do not think either side has yet formulated a compelling case. For instance, a reader writes in with the following question:

I've been hearing a lot lately that since Hillary won battleground states of OH, FL, and MI (and will probably win Pennsylvania) then she is better positioned to win those states in the general election. But is this actually true?

The answer is not necessarily. Clinton's flacks are commiting a fallacy of composition. The primary electorate is a subset of the general electorate. Most of those who vote in the primary are extremely likely to vote in the general, and are also likely to support the Democratic nominee. That is what partisanship is all about. So, what the Clinton advocates are assuming is that because partisan Democrats in a given state support Clinton over Obama - the entire state will. This need not be the case. It could just as easily be that Independents and persuadable Republicans actually prefer Obama to Clinton in the swing states - so, in an ironic twist, Clinton can win the primary but cannot win the general.

Obama's supporters make their own mistakes. For instance, they argue that his strength among "Independents" and "Republicans" is evidence of an advantage in the general. By itself, this is not. The only data we have on these voters is their self-identification. We do not have histories of how they voted in general elections. All we know is how they voted in this primary. To make the point Obama's supporters wish to make, they would have to show that these self-identified Republicans and Independents are typically reliable votes for Republicans in the general. They might be reliable Democratic supporters who see themselves as Republicans. They might be solid Republicans who are attracted to the high profile Democratic race but who nevertheless will come home to McCain in November. The fact that they voted for Obama in a primary is not determinative of what they typically do in November, or what they will do if Obama wins the nomination.

Now - here's the big caveat. The point is not that these arguments are untrue. It is, rather, that they are underdetermined. Personally, my intuition is that both might be correct. But that's all I have right now - intuitions. These are theories that are plausible, and might be worthy of testing if data were available (I am working on the Clinton camp's argument right now).

Of course, both sides - and many in the media, for that matter - are pointing to the relatively high turnout in the Democratic primaries as evidence of a Democratic advantage in the general. Roughly 62.2% of all primary votes have been cast in the Democratic primary. This is an impressive statistic. However, it cannot count as evidence for a Democratic advantage. The reason is that Democrats typically out-perform Republicans in the primaries. The following chart reviews this point by comparing the Democrats' share of primary turnout against their share of the two-party vote in the general election.

As you can see - 62.2% is far from extraordinary. Even when we exempt the years in which the Republican Party had a non-competitive contest (1972, 1984 and 2004), we see that the Democrats typically out-perform the GOP. Pulling in 62.2% of the primary vote is no unique feat for the Democrats.

The key year to examine is 1988. This is the best apples-to-apples comparison of 2008 that there is. That year, both parties had open, and very competitive, primary nomination battles. The Democrats out-performed the GOP by a margin larger than what they have done this year, pulling in a little more than 65% of the total-primary vote. Did it do them any good in the general? No. George H.W. Bush beat Michael Dukakis, 54 to 46.

Why does this argument not hold up? There are two reasons. First, it takes the higher turnout as a real-world example of the enthusiasm gap found in the polls. The above chart indicates that this is hasty. While there does seem to be an enthusiasm gap - this 62.2% cannot be taken as evidence of it because it is consistent with years in which there has not been such a gap. Typically, Democrats have had more exciting primary battles than Republicans, which is one reason that the above graph shows stronger Democratic turnout this year. The same goes for 2008. The Democratic race is simply more compelling than the Republican one. This might be drawing people out to participate - which would give the Democrats an edge in turnout regardless of how enthusiastic the base is.

Second, even if we cede that the Democratic base is more enthusiastic - and indeed, I think we should - enthusiasm only has a limited effect in American elections. If Voter A can't decide whom to support on Election Day, and breaks the tie by flipping his coin - his vote counts for exactly one vote. If Voter B is so excited to support his beloved candidate that he can't sleep the night before, and he actually weeps as he casts his vote for this savior-politician, his vote counts for exactly...one vote! We do not weigh votes according to how enthusiastic the voter is.

Enthusiasm can make a difference in the relative likelihood that Voters A and B will vote. Voter B is almost assured to vote, Voter A is much less likely. That is the benefit of enthusiasm. However, the likeliness of Voter A actually voting increases as the competitiveness of the contest increases. This is why, for instance, turnout was down 8 million votes between 1992 and 1996, was back up in 2000 and even higher in 2004. What's the difference? 1992, 2000 and 2004 were intense elections that captivated the national consciousness.