From a recent selloff in the shares of Nvidia Corp., to Oracle Corp.’s plunge after reporting mounting spending on AI, to souring sentiment around a network of companies exposed to OpenAI, signs of skepticism are increasing. “We’re in the phase of the cycle where the rubber meets the road,” said Jim Morrow, chief executive officer of Callodine Capital Management. The S&P 500’s three-year, $30 trillion bull run has largely been driven by the world’s biggest tech companies like Alphabet Inc. and Microsoft Corp., as well as firms benefiting from spending on AI infrastructure like chipmakers Nvidia and Broadcom Inc., and electricity providers such as Constellation Energy Corp. If they stop rising, the equities indexes will follow.