Pew Research: “Those who study politics have long known that a person’s party affiliation is a strong predictor of how they will vote and what their opinions will be on most political issues. Some of the power of partisanship comes from its relative immutability: Most people remain loyal to a political party.”
“But over a 15-month period encompassing the 2016 presidential campaign, about 10% of Republicans and Democrats ‘defected’ from their parties to the opposing party.”