As the one true known quantity in the Democratic nomination race, former vice president Joe Biden sits atop the pack of 2020 presidential contenders, relishing the prime position as he hosts a kickoff rally Saturday. No one knows whether the man who served as number two to popular Democratic president Barack Obama for eight years will run away with this contest — his third White House bid in as many decades — or fade out in the months-long test of political skill and stamina to come. After a month of modest events at union halls and pizza joints in early-voting states like Iowa, Biden is counting on making a splash at a rally in Philadelphia, the largest city in must-win Pennsylvania, a state Trump snatched from Democrats in 2016.