Via Mike Pridmore, Rhodes Cook notes that primary turnout is a terrible indicator for general election results.
The sample size of Presidential races is way too small to draw any real conclusions. Here's Cook.
The Democrats in particular have had a number of "negative" high turnouts, where friction between various wings of the party produced substantial voter interest but a badly scarred nominee with little chance of winning the general election.
It happened in 1972, when the controversial anti-Vietnam War campaign of George McGovern barely prevailed over more moderate elements in the party. It happened again in 1984, when former Vice President Walter Mondale could not shake off primary challenges from Gary Hart and Jesse Jackson. And to a degree, it happened a third time in 1988 when Michael Dukakis, Al Gore and Jackson kept fighting weeks beyond that year's large Super Tuesday vote before Dukakis finally nailed down the Democratic nomination.
In contrast, the 2000 Republican contest between Bush and McCain arguably produced a positive turnout surge. The two candidates battled across the February calendar that year and into March, setting GOP primary turnout records in far-flung contests from New York to California. Yet in spite of the intensity of that campaign, it ended amicably enough on Super Tuesday. McCain abandoned his candidacy in favor of Bush, and the Republicans marched united into a fall campaign which they ultimately won.
Given that base Democratic voters (as opposed to activists and elites) like both Clinton and Obama, and Republicans are not especially fond of John McCain, it seems like a united Democratic Party and a depressed and damaged GOP will be the outcomes. Obama is kicking the crap out of McCain on the money side in a way that is probably unprecedented; Obama will have a billion dollars to spend, and McCain will have much less than that.
I wonder if fundraising - either number of donors or total amount - is a proxy for general election strength. Bush outraised Gore substantially in 2000, and the conservative movement was much more energized at that time than the progressive base. I think Clinton beat Dole in 1996 by total amount, but I'm not sure about the number of donors. And in 1980, Reagan's direct mail base was substantially larger than Carter, though this may not have been true in 1972, when McGovern's campaign pioneered direct mail fundraising.