The following article by Philip Bump was posted on the Washington Post website July 24, 2018:
There has been an effort, often by Republicans skeptical of President Trump’s administration, to explain why Trump consistently sees approval ratings among members of his own party that are so much higher than past presidents. We noted in June that the only Republican president with more support from Republicans in the modern era was George W. Bush — immediately after the Sept. 11 terrorist attacks. How could that be, many have wondered, given how polarizing Trump is? There are certainly reasons for Republicans in particular to support Trump’s presidency: the strong economy, his appointments to the courts. But so much more than past Republican presidents?
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.