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Why is Trump so much more popular with Republicans than past presidents?

President Trump turns to depart after speaking in the Hall of Columns as he arrives on Capitol Hill on June 19 to rally Republicans around a GOP immigration bill. (AP)

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.

Those data are rough. Charles Franklin, director of the Marquette Law School Poll, walked through an analysis of partisan identification trends on Twitter. The effect of a decrease in Republican identity under Trump would be small, Franklin argues, because that shift has been small. FiveThirtyEight’s Nate Silver disagrees but is skeptical that looking at the big picture will answer the question anyway.

So let’s try.

The General Social Survey, a study of cultural and political attitudes conducted by the National Opinion Research Center at the University of Chicago, shows a steadiness in the percentage of self-identified Republicans over the past decade or so that’s similar to Pew. The percent of Americans identifying as Democrats has increased.

But that chart groups independents who generally vote with a party as members of that party. If we exclude those “leaners” from that chart, the picture for the Republicans is a bit worse. Since a peak in 1992 at 32.6 percent, the density of strong and not-strong Republicans has slipped back into the low 20s — where it was before the presidency of Ronald Reagan.

The downward trend in Republican identity has been fairly steady since 2004. At the same time, the percentage of Americans who say they’re independent but lean to the GOP has been remarkably consistent in the survey, holding at about 10 percent.

This data ends in 2016, at the very start of the Trump era. Gallup’s monthly polling since January 2016, though, shows very little change in identification by party over that period.

The takeaway, then, is that the density of those identifying as Republican fluctuates a lot but hasn’t changed dramatically recently after a slow slide beginning in Bush’s second term.

But the other part of our original theory is nonetheless accurate: On political ideology, the long-term change in both parties has been significant.

We’ve pointed to this graph before, most recently when noting that any attempt by the Democratic Party to stave off a liberal insurrection was coming too late. Since 2000, the number of self-identified conservatives in the Democratic Party has been cut in half. The number of self-identified liberals — only slightly more than conservatives 18 years ago — now constitutes half the party.

The General Social Survey lets us see the trend over a longer period. More than half of the most stalwart Democrats already identify as liberal after a slow climb upward.

Among that group, almost one in eight identify as “extremely liberal.”

As the chart of Gallup data above suggests, the percentage of Republicans identifying as conservative is much higher, as is the percentage of strong Republicans who identify as extremely conservative.

The GSS data mirror the Gallup shifts, but we can look at it over a longer period of time.

The Republican Party is much more ideologically pure than the Democrats — and than the Republican Party of 18 years ago. Then, 39.5 of Republicans identified as conservative or extremely conservative; now, 47.7 percent — almost half — do. Only 6 percent identify as liberal.

We can see that movement in Pew polling on policy issues. Note the split that occurs over time.

Trump, too, has grown more conservative in recent years, and his presidency reflects that. He’s made goading the political opposition a central part of his strategy in office and it remains a core part of his appeal. Whether the party is shedding more moderate members at a significant rate, it’s obviously true that the party’s ideology has become more homogeneous than it was in the past.

A president who appeals to that homogeneity is likely to fare well.