U.S. stocks headed for their biggest drop since mid-March after President Joe Biden was said to propose doubling the capital gains tax to near 40% for the wealthy. The dollar rose.

The S&P 500 turned lower after Bloomberg News reported the likely proposed plan on how investment income is taxed. Speculation arose that some investors may sell shares before any change is made to capture the lower rate. Equities whipsawed throughout the session amid mixed economic data and renewed concern the pandemic was worsening in some major economies.

All major groups in the S&P 500 fell, led by commodity and technology shares. AT&T Inc. rallied after beating profit estimates. Data Thursday showed sales of previously owned U.S. homes slid in March to a seven-month low, while jobless claims posted an unexpected decline last week.

“Sticker shock over some of these tax figures will be hard to shake off for some investors,” Edward Moya, senior market analyst at Oanda Corp, wrote in a note. “Some traders are looking for an excuse to lock in profits and they might choose to use this tax story as their catalyst.”

A gauge of market breadth suggests investor concerns remain. The percentage of Russell 2000 members trading above their 50-day moving average fell below 40%, compared to about 80% for the large-cap S&P 500, a smidgen off a record gap, according to data compiled by Bloomberg going back to 1995. It’s too early to tell whether this can morph into something larger, but previous instances have led to subsequent pressure on U.S. stocks, wrote Sundial Capital Research founder Jason Goepfert in a note to clients.