In 1896, conservative Democrats loyal to the outgoing incumbent, Grover Cleveland, condemned Bryan’s talk of bashing big business and even mounted a third-party ticket they knew would help the Republican nominee. In 1972, the A.F.L.-C.I.O., headed by George Meany, blasted McGovern as “an apologist for the communist world” whose delegates were “kooks and nuts.” After McGovern was nominated, the labor body, the indispensable pillar of the New Deal coalition, refused to endorse anyone for president. The decision broke a tradition of backing Democratic nominees that stretched back almost four decades.

At the 1964 Republican convention, Goldwater’s “extremist” admirers loudly booed his moderate critics, and the intraparty bitterness provoked many lifelong Republicans to vote for Johnson that fall. For his part, Jesse Jackson, despite winning over a thousand delegates in 1988, came in second for the nomination to Michael Dukakis, whose bland rhetoric and cautious promises contrasted sharply with his rival’s rousing style and left-wing policies.

In his acceptance speech that summer, Dukakis declared: “This election isn’t about ideology. It’s about competence.” That line not only failed to win him enough votes to prevent the Republicans, under George H.W. Bush, from winning a third straight easy victory. It also betrayed a fundamental misunderstanding of how major changes have always occurred in our country.

Americans who are seriously disenchanted with an incumbent president or his party tend to be moved more by a serious candidate who offers a sharply different alternative, one based on a set of moral convictions, instead of merely a sense of who might be a more efficient administrator of the existing order. Such voters are usually most numerous among the young. After their candidate loses, the fervent hopes he (and someday, she) inspired continue to motivate followers to convert their party to the same ideas and chart a path to future victory.

Since he began running for president five years ago, Senator Sanders and his supporters have nudged Democrats to take stands to the left of where the center of the party was when Barack Obama moved out of the White House. Every remaining candidate for president now endorses either Medicare for All or a robust public option, doubling the minimum wage, much higher taxes on the rich, legislation to facilitate union organizing and a transition to an economy based on sources of renewable energy. Even if the delegates in Milwaukee this summer choose a different nominee, they will surely endorse such policies and make them central to the drive to make Donald Trump a one-term president.

So whatever his electoral fate, the socialist from Vermont who is pushing 80 is likely to be remembered as a more transformative figure than many politicians who won an election but whom most Americans were quite glad to put behind them. Mr. Sanders wants to be the next Franklin Roosevelt — but if he can’t, better to be the next William Jennings Bryan or Jesse Jackson than the next William Howard Taft.

Michael Kazin, a professor of history at Georgetown and a co-editor of Dissent, is writing a history of the Democratic Party.

The Times is committed to publishing a diversity of letters to the editor. We’d like to hear what you think about this or any of our articles. Here are some tips. And here’s our email: letters@nytimes.com.

Follow The New York Times Opinion section on Facebook, Twitter (@NYTopinion) and Instagram.