Michael Barone is one of the most astute political analysts around. For one thing, he meticulously studies data, such as demographics and voting patterns.
Here he shows that on the whole, Hillary Clinton does well among what Barone calls "Jacksonians," basically the people that never watch PBS and work at real jobs if they are employed. Many of them are Scotch-Irish--that's a whole nother discussion. Obama does well among blacks, of course, but also among "Academics," either folks who actually are affiliated with universities, or people who graduated from one or are employed by the government. The pattern is pretty consistent, except for a few places, such as the Wisconsin primary.
The Jacksonian candidates have usually won the Democratic nomination, and when they didn't, the candidate got clobbered (Adlai Stevenson, George McGovern). This year, it looks like the Academic candidate, Obama, may win, aided by his near-unanimous support among blacks.
This analysis does not portend well for Obama in a race with McCain, who is Scotch-Irish by descent, and can cut into the Dems' vote. Of course, Obama has the war (depending partly on how it goes) and the economy to help him.
One partial answer to Obama's dilemma--Jim Webb for VP. He may be a stiff, but the Jacksonians are his people.