This is an excellent short article by the Pew Research Center (PRC). PRC reports that immigration policy has been a focal point of Trump’s campaign since he announced he was running for president. This article explores where Trump’s supporters stand on key policy issues.
Source: 5 facts about Trump supporters’ views of immigration | Pew Research Center
Apart from explaining why Trump is so popular, the results will potentially be generalizable and of interest internationally, especially in Europe where many countries are seeing a rise in populism as a backlash against immigration.