WASHINGTON — Senate Democrats, aware of the dead weight that Donald J. Trump has placed on their vulnerable Republican colleagues, can taste a reclaimed majority. But just as Senate Republicans blew their chances in 2010 and 2012 before finally taking control in 2014, Democrats find themselves hobbled by less-than-stellar candidates in races that could make the difference in winning a majority. In Pennsylvania, Katie McGinty, a relatively unknown former federal official who has never held elective office, is ahead in polls but lags Hillary Clinton’s large lead in the state. In Florida, a nasty primary between two flawed candidates could...
