It is not yet Labor Day, but the attack ads are already on the air.

Though Sens. Barack Obama and John McCain have said they want to change the way campaigns are conducted, neither presidential candidate appears likely to accomplish that goal. There seems to be some deep, structural feature of our electoral process that compels the campaigns, even against the wishes of the candidates, to turn to the dark side.

What is it?

One theory, popular with Democrats, is that negative campaigning was invented in the Reagan era by Roger Ailes and Lee Atwater, and later perfected by Karl Rove. One problem with that theory is that it omits mention of Democrats, such as James Carville, who have also excelled at the black arts of negative campaigning. And it forgets that it was the Democrats who in 1964 ran the most brilliant and brutal negative ad of all time, the "Daisy" ad, which implied that failure to support Lyndon Johnson would lead to nuclear war (check it out at www.youtube.com/watch?v=OKs-bTL-pRg, if you've never seen it).

The real problem with that theory, however, is that it doesn't dig deeply enough. It holds that campaigns go negative because negative campaigns win elections.

But what we really need to know is why negative campaigns win.

Here's my counter-intuitive suggestion: the problem is education. As the electorate has become better educated, we have also become increasingly ideological. And the bitterness of our political climate is a side effect of ideological polarization.

American politics has always been partisan, and nasty barbs have always been exchanged between candidates representing the two major parties. But, historically, campaigns did not generate sustained feelings of bitterness and alienation, because our political parties were pretty much broad coalitions representing a variety of regional and economic interests.

In less ideological times, being a Republican or Democrat was like being a Red Sox fan or a Yankees fan is today -- an identity one pretty much inherited and one unlikely to change from youth to adulthood. Elections in earlier days raised passions like those we now associate with professional sports, and partisanship was more fun than bitter. In the heat of a close contest, we might say that the Yankees or their fans are evil, but at some level we know it's not really true. They're just the other team, and their fans are rooting for their team because it's their home team.

A great issue like slavery or a great event like the Depression might disrupt these allegiances and cause a fundamental realignment of the party system, But, for the most part, partisan politics historically was not terribly ideological or terribly bitter.

Since the 1960s, however, the two political parties have become increasingly defined by ideology.

It is striking that voters with the most education tend to be the most likely to hold some political ideology such as liberalism or conservatism, and they are the most likely to favor extreme positions (for example, that abortion should always be allowed and funded by the state or that abortions should never be permitted).

Ideologies are useful to the extent that they help us make sense of complex issues and events. But they can also be blinding, warping our view of the facts and inhibiting our ability to sympathize with others who disagree with us.

Ideology makes us see people who disagree with us not as people who happen to root for the other team (New Yorkers cheering on the Yankees), but as wrong, dangerous and evil. If you're a Democrat, that's probably what you feel about the Bush administration, and it's what many Republicans felt about the Clinton administration.

Sadly, it also turns out that the ideologically committed are much more likely to vote than middle-of-the-road moderates.

The result: a tense and bitter political climate.

Voters who believe the other party is a force for evil do not generally want to see generous ads about the nobility of the opposition. They hunger to see their opponents castigated for their wickedness and to have their own moral superiority reaffirmed.

What Karl Rove realized is that in this ideologically polarized environment, a candidate who could mobilize his own most ideologically committed supporters would be more likely to win than one who tried to assemble a broad and diverse coalition.

That's why the campaigns of 2000 and 2004 were so bitter, and why the bitter politics of moral superiority is likely to remain with us for quite some time.

Joseph R. Reisert is associate professor of American Constitutional Law and chairman of the Department of Government at Colby College in Waterville.