When Democratic Sen. Chuck Schumer surveyed the political landscape a few months before the election, it must have looked like a dream about to come true: Hillary Clinton would win the presidency in a breeze. Democrats would retake the Senate. Mr. Schumer, 66 years old, would be majority leader. The dream died on Nov. 8. Donald Trump won the White House and Republicans held the Senate, leaving Mr. Schumer with the lesser post of minority leader. Now instead of leading Democrats to new progressive heights, his task is to protect President Obama’s legacy from being wiped out.