Here's a good example of why this approach is incorrect from TNR's Josh Patashnik:
John King on CNN just pointed out that Clinton did better in more Republican parts of Mississippi, and implied that this might mean that Clinton would do better among Republicans in November. This is patently the wrong conclusion to draw, and is a good demonstration of the perils of attempting to predict general-election outcomes based on primary results. Obviously, whites voted heavily for Clinton, and white areas of the state support Republicans in presidential elections--thanks to the votes of people who don't vote in Democratic primaries. Clinton's strong showing in these parts of the state doesn't demonstrate anything about the preferences of a single Mississippi Republican. This is a simple enough point that CNN should be able to convey it to viewers.
Jay Cost makes a similar point in his blog on Real Clear Politics:
[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?
Not necessarily. Clinton's flacks can indeed be seen on the airwaves arguing this point - but in so doing they are committing an inferential error. What they are assuming is that because partisan Democrats (her core support group) 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 would prefer Obama to Clinton in those swing states. So, in an ironic twist, Clinton would win the primary but not the general. Perhaps the Clinton campaign wishes to argue that Obama could not win in the general the voters she has won in the primary. Maybe - but the primaries alone do not indicate whether that is the case.
Obama's supporters have made their own errors. For instance, one can often find them arguing that his primary strength with self-identified Republicans is evidence of an advantage in the general. By themselves, primary results cannot indicate this. The only data we have on these voters is their self-identification. We do not have histories of how they voted in general elections. To argue that Obama's margins among self-identified Republicans is a sign of strength in the general, they would have to show that these voters are typically reliable votes for Republicans in the general who are being wooed away from their party. We cannot assume that this is the case. Remember that there is always a portion of each party that votes for the other side. So, these voters might actually be reliable Democratic supporters who see themselves as Republicans (lots of people see themselves that way..."I vote for the person, not the party" - but it always seems to be that the better person is of the same party!). In that case, their support for Obama does not necessarily portend general election strength.
As Cost points out, it's also not true that the higher Democratic turnout in the primaries (relative to Republicans) implies anything about general election outcomes:
Even when we exempt the years in which the Republican Party had non-competitive contests (1972, 1984 and 2004), the Democrats typically out-perform the GOP. Pulling in 62.2% of the primary vote [the current total for Democrats] is no unique feat for the Democrats. 1996 is telling. Bill Clinton had no serious challenge while Bob Dole faced a protracted battle against multiple opponents. And yet the GOP still only pulled in 55% of the primary vote.
Another key year is 1988. This is the best apples-to-apples comparison of 2008 that there is. That year, both parties had open 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%.
Finally, smart pundits like Noam Scheiber and Yglesias are speculating that Obama could put states like Kansas, Montana, and the Dakotas in play, but that's just unrealistic. Obama's primary support in "red states" and his appeal to independents doesn't mean he's going to put heavily Republican states into play in the general election, as his supporters have suggested:
Mr. Obama has been toting up his victories to suggest a striking range of popularity in states that usually fall outside the Democratic electoral map. Yet though these states have helped give him a lead in pledged delegates, it appears far from likely that he would be able to carry some of them in a general election.
Kansas, Mississippi and Wyoming, for instance, tend to be reliably Republican states for that party’s presidential nominees. And his big victories in Alabama, Georgia, Louisiana and South Carolina were based in large part on black Democrats coming out in droves to support an African-American candidate.
“Most of those states haven’t voted Democratic in a presidential since the Johnson landslide over Goldwater in 1964, and we don’t see that changing,” said Harold Ickes, a senior adviser to Mrs. Clinton. “They’re great states, but Idaho, Nebraska and the Carolinas are not going to be in the Democratic column in November. He’s winning the Democratic process, but that is virtually irrelevant to the general election.”
Look deeper, the Obama campaign contends, and there is another argument in his favor in traditionally non-Democratic states: He has drawn more votes from independents and Republicans than Mrs. Clinton has. But it is unclear whether independents and some Republicans in swing states or Republican-leaning states would choose him over Senator John McCain of Arizona, the presumptive Republican nominee.
The list of battleground states may change on the margin, but in the end it's going to look pretty similar to 2004. And if Obama's the nominee, he'll be focusing all his resources on those states in the end just like every previous nominee.