But that’s not the whole explanation: Among voters over age 30, she won those making more than $100,000 by a 31-point margin, more than twice her 14-point lead among those making less.

The same story was clear in pre-election polling, both in Iowa and elsewhere.

Her strength in affluent suburbs around Des Moines flipped the Des Moines metro area, which went strongly for Mr. Obama in 2008. It probably played a big part in suppressing Mr. Sanders’s margins in college towns like Iowa City, where Mr. Sanders might have been expected to run as well as Mr. Obama or even better, given his tremendous strength among young voters.

Mr. Sanders’s weakness among affluent voters is potentially a bad sign for his chances. In 2008, voters making more than $100,000 a year represented 25 percent of the national electorate but just 19 percent of Iowa caucus-goers, suggesting that the state may be a relatively good one for a candidate, like Mr. Sanders, who does best among less affluent voters. On Super Tuesday, the most liberal states happen to be particularly affluent ones, like Massachusetts, Colorado, Virginia and Minnesota.

But there is a bright side for Mr. Sanders: To some extent, he compensated for his losses among affluent voters by doing best among lower-income voters. He won white voters making less than $50,000 by nine points, 53 to 44 percent. Mr. Obama had lost white voters making less than $50,000 per year by two percentage points in 2008. Mr. Sanders was nearly as competitive as Mr. Obama across rural Iowa. He even won along the border with Nebraska — the relatively conservative, western part of the state that was basically the one place Mrs. Clinton won in 2008.