Claiming the mainstream . . . wherever that may be

Today is the first day of the Democratic National Convention in Boston, so it seems appropriate to think about something both parties would have us believe: that each of its presidential candidates claims to speak for "the mainstream" of American society. The only questions I have are, what exactly IS the mainstream? How is it determined, and who gets to determine it?
One method is presented in this op-ed piece in today's NY Times (be sure to click on the multimedia link). The visual is useful in showing that on the liberal-to-conservative continuum, Kerry is a bit more liberal than Bush is conservative and/but each is to the left (Kerry)/right (Bush) of his respective party's "median." But, though the piece explains the methodology used to produce the visual, it still begs the question of just how its writers define the terms "liberal" and "conservative." Its writers seem to presume that we understand those terms implicitly, but I personally would like to see a bit more clarification--and not just from the writers of this piece. I'm not playing dumb here: these terms get used as though they were black-and-white terms, and they clearly aren't, as the chart itself shows.
I think that something else the chart shows is far more telling: that we're seeing the BANKS of the mainstream but not the stream itself. The parties have polarized and thus the legislative process has become paralyzed by ideological name-calling and undue emphasis on winning at the expense of representation. We saw this most clearly during the recent debate on the proposed constitutional amendment that would define marriage as the union of one man and one woman. Poll after poll shows sizeable majorities of Americans as being opposed BOTH to same-sex marriage AND to a constitutional amendment that would effectively ban it; those who introduced the amendment knew they didn't have the votes even to continue debate on it; the sole purpose in introducing the amendment was to put senators on record as supporting or opposing it. The threat by some gay activists to out closeted gays working in support of the amendment is just another version of what the amendment's supporters are guilty of. The point here isn't to discuss the merits or demerits of the issue of same-sex marriage but to argue that whether the proposed amendment reflected the people's will was, at best, a secondary matter and thus unfair to the issue itself, cheapening its significance for all of us, no matter our opinion about it.
Thus: the mainstream gets defined by people who aren't in it. And so it is, then, that this article in The Onion might hit closer to the truth than the Paper of Record.

Addendum: "Mainstream" connotes kineticism, a dynamic; the banks of the mainstream something rather more static . . .