The S&P 500's energy sector was suffering a unanimous selloff, as crude oil /zigman2/quotes/209723049/delayed CL00 -0.94% and natural gas futures /zigman2/quotes/210189548/delayed NG00 -1.44% slumped amid growing demand concerns and as the Biden administration calls for a gas-tax holiday. The SPDR Energy Select Sector ETF /zigman2/quotes/206420077/composite XLE +0.76% dropped 3.4% with all 21 equity components losing ground, as crude oil futures shed 4.4% and natural gas futures lost 1.0%. Futures /zigman2/quotes/209948968/delayed ES00 -0.29% for the S&P 500 /zigman2/quotes/210599714/realtime SPX +1.73% were down 1.4%. The energy ETF's biggest decliners were shares of Marathon Oil Corp. /zigman2/quotes/205031829/composite MRO +0.63% , down 5.2%, and Devon Energy Corp. /zigman2/quotes/209479244/composite DVN +1.69% , down 4.5%. Among other more-active components, shares of Occidental Petroleum Corp. /zigman2/quotes/207018272/composite OXY +0.06% slid 3.4%, Exxon Mobil Corp. /zigman2/quotes/204455864/composite XOM +0.87% declined 3.0% and Kinder Morgan Inc. /zigman2/quotes/208455654/composite KMI +0.54% shed 2.6%. The best performer was Williams Companies Inc.'s stock /zigman2/quotes/205467183/composite WMB +0.65% , which fell 1.9%.