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 harm the Democrats’ chance to unseat Senator Marco Rubio.

Several high-profile Democrats turned down the chance to challenge Senator Richard Burr of North Carolina before they settled on a civil liberties lawyer, Deborah Ross, who is not necessarily a good fit for suburban voters there. Catherine Cortez Masto, a Democrat and former state attorney general now running for an open seat in Nevada, has also failed to catch fire.