The following article by Philip Bump was posted on the Washington Post website July 24, 2018:
One popular theory is that the increase in self-identified independents reflects a movement away from an increasingly partisan Republican (and Democratic) Party, consolidating a more-extreme base within the party that approves of Trump’s actions.
While it’s true that the number of partisans has increased, it doesn’t seem to be true that a significantly lower percentage of Americans is identifying as Republican in the Trump era relative to past years. Pew Research Center polling shows that the biggest drop in Republican Party identity came in the last few years of the Bush presidency, as an unpopular president turned off voters. In 2009, the percentage of the country that identified as Republican was 26 percent; in 2017, it was the same.