AUG. 25, 2016

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 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.

To challenge 82-year-old Senator Charles E. Grassley of Iowa, Democrats settled on 72-year-old Patty Judge. Senator Rob Portman’s Democratic challenger in Ohio, former Gov. Ted Strickland, is 75, an easy target for Mr. Portman’s taunting nickname, “Retread Ted.”

The Democrats’ problem stems from a depletion of their ranks in state legislatures and governors’ mansions over recent years and a lack of institutional support for grass-roots-level politicians who represent a changing base.

“Democrats cannibalize each other when they lose those seats and don’t have new talent to fill them,” said Daniel A. Smith, a professor of political science at the University of Florida. “Here and in Pennsylvania and Wisconsin and North Carolina are states that should have Democratic state-controlled legislatures, and the fact that they don’t not only marginalizes Democrats, but also makes it increasingly hard to build a farm team.”

Republicans, of course, find themselves in a fundamental conflict between Mr. Trump’s populist insurgents and traditional conservatives. But Democrats are mired in their own struggle, as they try to identify future stars who can appeal to a base increasingly insistent on a progressive agenda.

Florida’s Senate Democratic primary this Tuesday pits a bombastic, populist liberal, Representative Alan Grayson, against the establishment’s pick, Representative Patrick Murphy, in the kind of showdown that analysts expect to see in the party’s future.

“Democrats are going to have their own Tea Party moment in 2018,” said Jennifer Duffy, a senior editor and Senate analyst for The Cook Political Report. “I don’t think they are going to put up with the party dictating who their candidates are.”

The issue was highlighted this year when Senator Bernie Sanders of Vermont found considerable success by running against the sort of incremental liberalism of President Obama and Mrs. Clinton.

Mrs. Clinton followed that pattern with her selection of Senator Tim Kaine of Virginia, who is to the right of many progressives, as her running mate.

While some up-and-coming Democrats like Senators Cory Booker of New Jersey and Kirsten E. Gillibrand of New York had speaking spots at the party’s convention last month, none had the prominence of Mr. Obama in 2004, when he gave the keynote speech that lifted him to national prominence.