By ZEKE MILLER and SEUNG MIN KIM WASHINGTON — Vice President Kamala Harris moved swiftly Monday to lock up Democratic delegates behind her campaign for the White House after President Joe Biden stepped aside amid concerns from within their party that he would be unable to defeat Republican Donald Trump. Biden’s exit Sunday, prompted by Democratic worries over his fitness for office, was a seismic shift to the presidential contest that upended both major political parties’ carefully honed plans for the 2024 race. Aiming to put weeks of intraparty drama over Biden’s candidacy behind them, prominent Democratic elected officials, party leaders and political organizations quickly lined up behind Harris in the hours after Biden announced he was dropping his reelection campaign. Biden’s departure frees his delegates to vote for whomever they choose.