The U.S. economy is increasingly running on a peculiar fuel source: the spending habits of rich people. And that's making some economists nervous about what happens if the stock market takes a dive.

Mark Zandi, chief economist at Moody's Analytics, recently told Bloomberg that high-income households represent the "last pillars" of strength in the economy. A stock market downturn would "knock the wind out of" them, significantly raising the risk of a recession.

Here's the thing: company executives and economists have been noticing a widening economic split, where wealthy Americans drive most economic activity while low-income households continue struggling. The numbers tell a striking story.

The Top 20% Are Carrying the Economy

The top 20% of earners now account for almost two-thirds of all spending in the U.S., according to Moody's data cited by Bloomberg. That's a record. Meanwhile, the bottom 80% used to make up nearly 42% of spending before the pandemic, but that share has dropped to just 37%.

"The 20% of households that make more have done much better, and those in the top 3.3% of the distribution have done much, much, much better," Zandi wrote in a September post on X. "The U.S. economy is being largely powered by the well-to-do. As long as they keep spending, the economy should avoid recession."

But what happens if they stop spending? The U.S. economy could face a "big problem" if wealthy Americans become cautious with their wallets, Zandi warned.

Why AI Stocks Matter More Than You Think

This is where the stock market becomes critically important. The richest 20% of U.S. households own nearly 93% of all stocks, according to data from New York University economics professor Edward Wolff cited by CNBC. When stock portfolios surge, wealthy households feel richer and spend more. When stocks tumble, they pull back.

"It's hard to overstate the significance of the soaring stock prices of artificial intelligence companies to the economy," Zandi said in a post on X earlier this month. "Spending by well-off Americans, driven by their surging stock portfolios, is the single most significant driver of growth."

That's what makes the growing concerns about an AI bubble and elevated tech stock valuations particularly worrisome. If those valuations come back to earth in a hurry, it could hit wealthy households hard and potentially drag the broader economy down with them.

The economy's dependence on affluent consumers creates a vulnerability that didn't exist to this degree before. When the people holding most of the stocks and doing most of the spending are the same group, a market correction doesn't just hurt portfolios. It threatens the main engine keeping the economy moving forward.