Since then, though, there’s been a steady shift on the left. In every single year save one, Gallup’s polling has found the percentage of Democrats who identify as liberal has either increased or stayed the same. In its most recent poll, released Tuesday, a new benchmark was set: More than half of Democrats now identify as liberals. By comparison, only about 1 in 8 identify as conservative.

(One interesting aspect of that graph worth noting: that little bubble of conservatism among independents in 2009 and 2010, a period that overlapped with the backlash to Barack Obama’s presidency and the rise of the tea party.)

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Why is that shift among Democrats particularly significant in this moment? Because it reflects a real change in the party that nominated Clinton in 1992 and which will next year probably nominate someone who will be very different politically from Clinton. We saw this emerge in dramatic fashion in 2016, when Hillary Clinton faced a harder-than-expected primary challenge from Sen. Bernie Sanders (I-Vt.). Sanders was much more representative of a more-liberal Democratic Party, from which he obviously benefited.

In 2017, the Pew Research Center found a widening gulf between Democrats and Republicans on a number of political issues over the same period — a widening gulf that revealed partisanship as the biggest indicator of political differences.

Why? In some cases because of sharper movement by Democrats. For example, Pew’s researchers write:

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For the first time, a majority of Republicans (54%) favor acceptance of homosexuality; just 38% did so in 1994. Yet over this period, the increase in the share of Democrats saying homosexuality should be accepted has been much larger (from 54% to 83%). As a result, partisan differences have gotten larger.

On a number of issues included in Pew’s research, the movement among Democrats was more significant than that among Republicans.

Included in Gallup’s most recent analysis is a breakdown of ideological identifiers by demographic. Besides Republicans, the group that most lopsidedly identifies as conservative is those 65 and older; among the most heavily liberal are those under 30.

Since Gallup has been conducting this research over a long time, we can compare those specific demographic responses from the 2018 polls to analysis conducted 10 years ago, in 2009.

Here is the shift in ideological identity from 2009 to 2018 by gender and age.

To some extent, as with those independents, the shifts are a function in the density of conservative identity as Obama took office. But the changes are still remarkable.

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For example: Those 65 and older were 32 points more heavily conservative than liberal in 2009 but only 21 points more conservative last year, a change of 11 points. The shift among those ages 30 to 49 was 21 points on net, from those identifying as conservative being a plurality to the smallest minority. Women saw a 14-point swing on net.

Part of the change among age groups is a function of people aging; 2018′s 30- to 49-year-old is 2009′s 21- to 40-year-old. But the shifts favored liberal identity across the board to some extent. Even among those under 30, where the density of self-identified liberals declined, self-identified conservatives declined more.

None of this means Sanders or Sen. Elizabeth Warren (D-Mass.) or another staunchly liberal candidate necessarily has an edge in 2020. But it is a reminder that the Democratic Party is not the hospitable home to moderates that it once was. For some segment of the Republican Party, that’s probably frustrating, particularly now that President Trump has redefined what it means to be Republican. More-moderate members of the party who bailed in the Trump era — like former New York Mayor Michael Bloomberg — have seemed to embrace the Democratic Party as a welcoming home, but such a welcome is unlikely.