A "Wall Street veteran" who once warned of a financial crisis is sounding a bubble alarm: U.S. stock valuations, leverage, and positioning are at the most extreme levels in his 40-year career.

David Rosenberg says the U.S. stock market increasingly resembles a bubble, with valuations, leverage and investor positioning at extremes he has not seen in his 40-year career. The founder of Toronto-based Rosenberg Research & Associates Inc. believes the current bull market is far more dependent on the trajectory of artificial intelligence (AI) progress than the stock market was on the tech sector on the eve of the dot-com bubble. He said he has been buying U.S. Treasuries and overseas stocks in his portfolio, and called the U.S. the "epicenter of the tech boom" for which investors are paying too high a price. "In my 40 years in this business, I don't remember seeing so much divergence, extremes and imbalances not just in the markets, but in the economy," he said at Grant's Annual Fall Conference in New York. "If you don't want me to call it a bubble, I won't. But what I'm seeing now is truly something I've never seen before." Rosenberg is known for his bearish stance. He was formerly chief North American economist at Merrill Lynch & Co. and rose to prominence after warning about the U.S. housing bubble before the 2008 financial crisis. He later spent more than a decade at Gluskin Sheff before leaving to found his own research firm. Beyond warning about the market's growing dependence on AI for further gains, Rosenberg also pointed to vulnerabilities in U.S. economic fundamentals. In particular, he noted that consumers are increasingly reliant on further stock market gains to fund their lifestyles. "I'm really worried about the social impact of the next bear market," Rosenberg said. Citing people who retired early and rely on their portfolios to sustain their lives, he said they will face difficult choices. "What happens to these people? Do they go to Walmart to look for a cashier job, only to find that even that job has been replaced by Siasun Robot&Automation?"