Mr. Sanders has voiced the concerns and energized millions of young people, many of them voting for the first time. His candidacy has forced the party to go deeper on addressing issues like wealth inequality, college tuition costs and the toll of globalization — important points of distinction with Republicans. What’s more, Mr. Sanders’s commitment to small individual contributions has put the lie to Democrats’ excuses that they, too, must play the big money game to win. This is a message too seldom heard in the party that first championed campaign finance reform. That it’s back is long overdue, good for Democrats and good for campaigning. Mrs. Clinton “is clearly irritated by the fact that she has to deal with this guy,” the Democratic strategist David Axelrod said in an interview. “But he’s pushed her on a lot of issues in a positive way, and I think that his young supporters will be bitterly resentful if anyone tries to shove him out of the race.”

Voters are keeping these also-ran candidates going. When Mr. Sanders loses in a state, he raises more money, not less. Voters consistently choose Mr. Kasich as the Republican most likely to beat Mrs. Clinton. This refusal to anoint a front-runner in either party appears in poll after poll, as dispirited voters declare that they simply don’t like Mr. Trump, Mr. Cruz or Mrs. Clinton. This should be a wake-up call to leaders of both parties. They are missing something big about their own members’ priorities, and their mood. A spirited nominating season might teach them what voters actually want from their president. So far, voters are saying they aren’t willing to settle for a party favorite, and don’t want to be cheated out of a choice.