For anyone closely watching the financial markets, the calm of Christmas on Tuesday was infused with anxiety.

Traders were bracing for the possibility of bad news on Wednesday, something bigger than just another day of stock market declines: If stocks don’t reverse their recent sell-off, the United States is poised to close the books on the longest bull market on record.

The major benchmark stock indexes are nearing losses of more than 20 percent from their peaks, a bear market’s official marker, which would end a nearly decade-long run of gains. From its high on Sept. 20, the S&P 500 is down 19.7 percent.

The market has reached this point after a rare downward streak this month — stocks fell for four consecutive days through Monday. Investors have looked past evidence of economic strength, however, and toward a potential slowdown in 2019.