To Democrats, the level of support for Trump within his party seems occasionally baffling. How could someone they hate so much be viewed so positively by the other party? Over the course of Trump’s presidency, a theory emerged: He’s so popular among Republicans because Trump-skeptical Republicans have simply given up on the party. Wring all the skeptics out of the party, and you’re left with a more unanimous, if smaller, core.

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There’s a public example of how this would work, after all: Had Rep. Justin Amash (I-Mich.) not left the party in July, the vote to impeach Trump in the House would not have been unanimously opposed by Republicans. Instead of Republicans voting 195-to-1 against impeachment, it was 195-to-0. This, perhaps, is how Trump’s approval also works.

Unfortunately for that theory, though, the numbers don’t really back it up.

On Tuesday, Gallup released its most recent assessment of the party identification of American adults. Twenty-eight percent identify as Democrats and 28 percent as Republicans. The rest are independents, though most of those independents end up voting consistently for one party or the other. Nine percent of Americans are what we might call “pure” independents, according to Gallup; another 15 percent are Democrat-leaning independents and 17 percent independents who lean Republican.

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The question, then, is how those figures compare to the beginning of Trump’s administration — or, perhaps, to the data from when he entered the race. Taking quarterly averages of Gallup’s polling, we can answer that question.

The numbers haven’t changed much.

Since Trump announced his candidacy in June 2015 (the second quarter of that year), there’s been a three-percentage-point jump in the number of people who identify as Republicans and a 2.5-point jump in the percentage of Democrat-leaning independents. Since his inauguration, the changes have been more subtle.

Overall, the distributions were about the same in the last quarter of 2019 as they were in the first quarter of 2017; that is, when Trump took office. There’s no evidence that Republicans are demonstrating an exodus.

It’s certainly possible that Republicans who approve of Trump had already left the party by the time he announced his candidacy. Gallup’s data begin in 2004, allowing us to look at the second term of George W. Bush’s administration. Over the course of those four years, the number of people who identified as Republican dropped by nearly eight percentage points. Since Bush was inaugurated for the second time, the density of Republicans in the public dropped more than seven points.

During Bush’s second term, Democratic density grew — including Democrat-leaning independents.

But once Barack Obama was elected president, the Democrats saw their own decline. During his eight years in office, the density of Democrats in the population sank by six points. The number of Republican-leaning independents grew by more than three points.

The story of partisanship in recent years is precisely that, of course: partisans becoming independents who tend to vote with parties. The Gallup data suggest that the Bush and Obama presidencies had the effect of wringing most of the less committed partisans out of their parties, shifting moderates from partisan identity to party-leaning independence.

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Remaining in the Republican Party, then, would be a denser core of committed partisans, even before Trump announced his candidacy. Although his nomination was far from a sure thing when he announced his candidacy, over time those Republicans came to embrace his campaign and presidency.