As Iowa Democrats streamed into their local caucuses Monday evening, they told pollsters by a margin of 61% to 37% that they would rather see the Democratic Party nominate a presidential candidate who can “beat Donald Trump” than one who “agrees with you on major issues.” But according to partial returns released Tuesday afternoon, their votes have made Sen. Bernie Sanders once again a leading candidate for the Democratic nomination.

It is difficult to believe that Mr. Sanders’s supporters consciously placed ideological commitment...