Mr. Kasich is not an exciting candidate, or even a political moderate. But he is the most sane-sounding individual in the Republican field, and has been from the start. Unlike his rivals, he’s shown a willingness to play by the rules. His presence in the race offers moderate Republicans a palatable alternative in the primaries and caucuses, and also if there is a real fight at the convention.

Mr. Sanders has always stood more for a vision than for reality, especially with a Republican-led Congress. As he and Mrs. Clinton tore into each other in last week’s debate in Brooklyn, some Democrats worried that the nasty fracas would hurt the party. Others want Mr. Sanders to get out and let Mrs. Clinton focus on the Trump threat.

Mr. Sanders’s presence has made this an immeasurably more substantive race, in which both candidates’ policies have been better vetted, and as a result, better delineated. That’s the best preparation for the general election. Yes, Mrs. Clinton’s lead is nearly insurmountable, but it should be voters who erase the “nearly.”

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.”