Republican activists in swing states say they have seen little sign of the teams tasked with knocking on doors and turning out infrequent voters on behalf of Donald Trump, raising concerns about the party’s presidential nominee relying on outside groups for an important part of his campaign operations. Trump and the Republican National Committee he controls opted to share get-out-the-vote duties in key parts of the most competitive states this year with groups such as America PAC, the organization supported by billionaire Elon Musk. It is difficult to demonstrate that something is not happening. But with fewer than 50 days...