Presidential hopeful Sen. Bernie Sanders (I-Vt.) will be running against Democratic candidates who have staked out similarly liberal positions on key issues. (Tannen Maury/EPA-EFE/Shutterstock)

The announcement by Sen. Bernie Sanders (I-Vt.) on Tuesday morning that he planned to seek the Democratic nomination for the presidency in 2020 had the effect of kicking a smoldering ember into a puddle of gasoline. In short order, Sanders had leveraged his robust, enthusiastic base from three years ago to raise some $6 million, according to the campaign, easily outpacing his announced 2020 competitors. It’s hard to see which other declared or possible candidates might even be able to challenge that pace of fundraising — including President Trump, who nonetheless decided on Wednesday to implore his supporters to do just that.

In one key way, though, Sanders’s 2020 run is already starkly different from his run in 2016: He’ll be running against candidates who — often following his lead — have staked out similarly liberal positions on key issues.

When Sanders first started to storm into contention in 2016, we noted that he was drawing strong support from the more-liberal wing of the party. While in the past that wasn’t a recipe for success for Democratic candidates, that Democrats had become so much more likely to identify as liberals by 2016 meant that Sanders had a strong base from which to field a candidacy.

Since 2016, that liberal constituency in the party has only grown. As we reported in January, the percentage of Democrats who describe themselves as liberal in Gallup polling moved past 50 percent for the first time in 2018. While Republicans are far more likely to identify as conservatives than Democrats are as liberals, Democrats are twice as likely to do so now as they were in 1994.

(Philip Bump/The Washington Post)

Gallup explored some of the underlying demographic shifts that are powering that change in an article this week. One of the most interesting shifts is seen in that first graph. White Democrats were 20 points more likely to identify as liberals from 2013 to 2018 than they were from 2001 to 2006. Black and Hispanic Democrats adopted that label at less than half the rate.

(Gallup) (Philip Bump/(Gallup))

We explored this in 2016, too, looking at another factor in Sanders’s candidacy: his soft support from black voters. Black Democrats hadn’t adopted the liberal descriptor as readily as whites, which might also help explain why Sanders wasn’t performing as well with that constituency.

Those lessons from 2016 are particularly interesting when we start looking forward. Sanders is running not against Hillary Clinton, a comparatively moderate candidate with a well-honed instinct to rock as few boats as possible. Instead he’s running against a number of his Senate colleagues, several of whom have, on at least one commonly cited metric, more liberal records than he does.

That metric is compiled by VoteView, which assigns members of Congress scores that, among other things, measure their liberal or conservative voting records. (In essence, this metric looks at support for government involvement in the economy.) Sanders is rated at -0.521, where 0 is more moderate and -1 more liberal. By comparison, Sen. Elizabeth Warren (D-Mass.) is rated at -0.76 — more liberal than Sanders on at least this metric.

Before we fight over that rating, let’s look at how Democratic fields have evolved over time in that regard. Below, we’ve positioned major Democratic primary candidates from contested cycles against the density of self-identified liberals in the party. (The gray box at the left includes those candidates who never served in office.) The darker blue dots indicate the eventual nominee.

(Philip Bump/The Washington Post)

(For those who no longer served in Congress at the time of their runs, we included the most recent year of VoteView data.)

Notice that the nominees in 2000, 2004, 2008 and 2016 were all rated fairly similarly on the VoteView metric. In 2004, the field was a bit more diverse, including, on the more liberal end of the spectrum, former Ohio congressman Dennis Kucinich (who also ran in 2008). That diverse field was less liberal on average than the one that elevated Barack Obama to the White House. Obama ran against a number of similarly rated peers that year, including Clinton.

There was some movement. In 2000, former vice president Al Gore was rated -0.316. In 2004, then-Sen. John F. Kerry (D-Mass.) was rated -0.352. Obama was -0.343 in 2008 and Clinton -0.367 in 2016. That’s not an insignificant change, but one that likely stemmed in part from Clinton’s voting record reflecting her eye on the White House.

Notice how Sanders compared to Clinton in 2016 — and where he is now. This year’s field as it stands has the most liberal average VoteView score, edging out that densely packed field in 2008. (2016′s was dragged to the center by former Rhode Island senator Lincoln Chafee.)

If, however, the field expands to include several possible candidates such as former vice president Joe Biden or former Texas congressman Beto O’Rourke, that average will drop. There are already several candidates in the race who are closer to the center politically, including Sen. Amy Klobuchar (D-Minn.); adding Biden and O’Rourke would bolster the number of more-moderate candidates. (Again: On this metric.)

The picture that emerges is one of diversity, with candidates covering a range of ideologies. At the more-liberal end, as you’ve noticed, are a group of candidates who are more liberal in VoteView’s eyes than any candidate who’s run since 1992 — a reflection of how some candidates are trying to leverage the more-liberal Democratic electorate. But that’s only part of the story.

In terms of VoteView scores, Sanders finds himself in a very unfamiliar position, at least among declared Democratic candidates: near the center. Not because he’s changed his positions, of course, but because the party has continued to grow more liberal — and because his competitors have to a large degree deliberately taken up the ideas that powered the enthusiasm of his 2016 base.