Stocks ended sharply lower Monday in a holiday-truncated session, putting key equity benchmarks on track to log their worst month and a year in a decade. The S&P 500 SPX, +0.29% slipped 2.7% to end near 2,352, based on preliminary numbers. The Dow Jones Industrial Average DJIA, +0.19% shed 644 points, or 2.9%, to end near 21,794. The Nasdaq Composite COMP, +0.36% was down 2.2% to finish around 6,193. The Dow recorded its fifth session with a 2% decline in December, its most in a month since Sep. 2011. Equities saw no respite after developments from Washington put concerns of political instability back on investors' agendas. In an attempt to ease investors' fears, Treasury Secretary Steven Mnuchin called CEOs of the nation's biggest banks over market liquidity. But his efforts may have amplified the market selloff. Investors also highlighted reports President Donald Trump had discussed the possibility of firing Federal Reserve Chairman Jerome Powell. Mnuchin, however, said Trump denied having made such remarks.