These little caucuses (Maine, Washington) attract the people mentioned by Riginslinger, but put more broadly, the people who are active in politics, canvassing for candidates, writing to the papers, attending political discussion clubs, kaffeeklatchers, eating and sleeping politics during election years- the activists.

Contrasting with Louisiana-
Although percentage relative to possible voters not very large, the Louisiana primary vote does seem large enough to be fairly representative.
Looking at Democrats alone, 358,000 turned out- and we are able to see the demographic splits in all their divisiveness.

[Louisiana- 4.5 million, 64% white. Last election, Gore 792,000; Bush 928,000 (total 1,720,000).
2008- 1,500,000 registered Democrats (but perhaps only 50-60% vote)]

2008 Primary Total- 358,000
Whites- 70% voted for Clinton
Blacks- 82% voted for Obama
Female- 60% for Clinton (exit poll figure)
Youth vote- Obama (can't find percentages)
Voters over 40- Clinton (   ")

For Maine and Washington, the numbers are too small to take the pulse of the State, and I can find no demographics on those attending (age, race, sex).