The former vice president has effectively been his party's leader since his last challenger in the Democratic primary, Bernie Sanders, ended his campaign in April.WASHINGTON — After primaries and caucuses in 42 states and the District of Columbia, Joe Biden has won the last few delegates needed to clinch the Democratic nomination for president as states worked to tally a surge of mail ballots.Indiana, Pennsylvania and Rhode Island were among the seven states, plus the district, holding elections Tuesday. But a huge increase in vote-by-mail ballots, driven in large part by the coronavirus...…The former vice president has effectively been his party's leader since his last challenger in the Democratic primary, Bernie Sanders, ended his campaign in April.WASHINGTON — After primaries and caucuses in 42 states and the District of Columbia, Joe Biden has won the last few delegates needed to clinch the Democratic nomination for president as states worked to tally a surge of mail ballots.Indiana, Pennsylvania and Rhode Island were among the seven states, plus the district, holding elections Tuesday. But a huge increase in vote-by-mail ballots, driven in large part by the coronavirus...WW…