The middle class is steadily eroding. Just ask the business world.

In response to the upward shift in spending, PricewaterhouseCoopers clients like big stores and restaurants are chasing richer customers with a wider offering of high-end goods and services, or focusing on rock-bottom prices to attract the expanding ranks of penny-pinching consumers.

“As a retailer or restaurant chain, if you’re not at the really high level or the low level, that’s a tough place to be,” Mr. Maxwell said. “You don’t want to be stuck in the middle.”…

While spending among the most affluent consumers has managed to propel the economy forward, the sharpening divide is worrying, Mr. Fazzari said.

“It’s going to be hard to maintain strong economic growth with such a large proportion of the population falling behind,” he said. “We might be able to muddle along — but can we really recover?”

Mr. Fazzari also said that depending on a relatively small but affluent slice of the population to drive demand makes the economy more volatile, because this group does more discretionary spending that can rise and fall with the stock market, or track seesawing housing prices.