Hillary Clinton and Donald Trump are in a statistical dead heat in Florida, where the state’s 29 electoral votes will be pivotal in Tuesday’s presidential election. One surprise for Democrats is that President Obama’s December 2014 decision to liberalize U.S. Cuba policy is not helping their nominee as the White House expected it to. Instead, it has become a liability.

Mr. Obama and Democrats bet big on the hypothesis that the traditional hard-line approach to dealing with the Castro regime, which encouraged the Cuban diaspora...