NEW YORK — With Election Day less than two months away, Democrats are increasingly worried that Hillary Clinton has not built a formidable lead against Donald Trump despite his historic weaknesses as a national party candidate. Even the Democratic nominee’s advisers acknowledge that she must make changes, and quickly. Clinton leads Trump by three percentage points, having fallen from her high of nine points in August, according to the latest RealClearPolitics average. That tightening has frustrated many Clinton allies and operatives, who are astonished that she isn’t running away with this race, given Trump’s deep unpopularity and his continuing stream...