The stock market was tumbling—again—Tuesday, with the S&P 500 back in correction territory following a massive turnaround Monday. All eyes were on the Federal Reserve as its two-day monetary policy meeting begins.

In morning trading, the Dow Jones Industrial Average was down 782 points, or 2.2%, one day after the benchmark reversed a 1,100-point loss to finish up 99 points. The S&P 500 dropped 2.7%, while the Nasdaq Composite slumped 3%.

With…