Should Mr. Sanders win the nomination and the White House, he would very likely inherit a Democratic Party whose numbers in Congress have sharply dwindled and whose proportions in state legislatures — the farm team for potential national officeholders — have likewise declined. In the 1960s and ’70s, when Mr. Sanders and Hillary Clinton came of political age, the Democratic Party embodied the power of politically active young people. Indeed, a number of current Democratic congressional leaders got their start in those years. In the current Congress, the oldest members of both the House and the Senate are Democrats; the youngest in both chambers are Republicans. Republican control of a greater share of offices at the state level has helped further the careers of younger Republicans like Marco Rubio and Ted Cruz.

Mr. Sanders’s own political career illustrates what can happen when a revolutionary has no ground troops. For 25 years in Congress, Mr. Sanders has held fast to his progressive message and principles. But he hasn’t gotten many big things done. As an uncompromising political independent, his outsider status has largely prevented him from attracting the support that would be needed among Democrats to turn into law his liberal ideals on health care, on college education and on fighting poverty and climate change.

One need only look to the legislative setbacks for President Obama to see what happens when transformative ideas hit an intransigent Congress. Mr. Obama’s campaign awakened large numbers of young people to politics, and his campaign network, like Mr. Sanders’s, operated outside the traditional party structure. After he won, a lingering economic downturn and rising distrust of government solutions propelled the most angry Tea Party elements of the G.O.P. to run for Congress. As a result, Mr. Obama has presided over the biggest loss of congressional Democrats in modern political history — 13 Senate seats and 69 House seats. Republicans now hold 31 governorships, many more than when Mr. Obama took office. State legislatures, too, have had a surge in Republican control.

The Democratic Party recognizes the problem, but whether it can alter the trend is another matter. Raul Alvillar, the national political director for the Democratic National Committee, says the party has demographics on its side, as a wave of young people reach voting age, and the party, through a series of training initiatives, is trying to inspire them to run for local office.