Trump’s power on social media, honed and enhanced through two national campaigns and nearly four years as president, gives Trump the opportunity to do something rarely attempted by the 44 men who previously held the nation’s highest office — keep the national gaze fixed on himself as a new chief executive tries to command the national stage. It’s somethinga force that Democrat Joe Biden, should he win the White House, will likely have to contend with as he seeks to reunify a nation fractured across regional, racial and partisan lines.