In a forthcoming paper, “Policy Over Party: Comparing the effects of candidate ideology and party on affective polarization,” Yphtach Lelkes, a political scientist at the University of Pennsylvania, reports that voters in primary elections prefer candidates who are willing to take more extreme positions.

“Partisans are reacting most favorably toward ideological extremists,” Lelkes wrote in an email:

In fact, moderates are punished for the policies. Extreme politicians in the vein of Mike Lee and Elizabeth Warren, are rated roughly 20-25 points higher (on a 0 to 100 scale) by party identifiers than ideologically moderate politicians in the vein of Susan Collins and Joe Manchin. These effects are even larger among ideologically extreme respondents — the kind that vote in primary elections.

Lelkes conducted experiments that asked participants to rate a number of hypothetical candidates, including a moderate Democrat, an “ideologically extreme” liberal Democrat, a moderate Republican and an “ideologically extreme” conservative Republican.

He found that the most ideologically committed voters — who are disproportionately represented in primaries and caucuses — strongly preferred more extreme candidates:

The impact of policy cues was particularly strong among respondents who themselves held strong policy positions. If a respondent was told that candidate held extreme in-party views, ideologically extreme respondents registered feeling thermometer ratings 35.89 points warmer than ideologically neutral respondents.

Lelkes’s conclusion?

If we are to decrease affective polarization in the United States, we need politicians that are politically moderate. Unfortunately, voters prefer politicians of their own party that are politically extreme. This incentivizes extreme political candidates, which will only exacerbate current tensions.

There are dangers for both parties is these trends, Lelkes argues, citing a 2015 study of House elections, “What Happens When Extremists Win Primaries?” by Andrew Hall, a political scientist at Stanford.

Hall found that when a more extreme candidate beats a moderate in the primary

the party’s general-election vote share decreases on average by approximately 9—13 percentage points, and the probability that the party wins the seat decreases by 35—54 percentage points.

Gallup data shows the steady ascendance within Democratic ranks of self-identified ideological liberals and a parallel decline in the share of Democrats who say they are moderate or conservative. From 1973 to 2018, as I have noted before, the percentage of Democrats who say they are liberal has grown from 25 to 51 percent, while the share of moderates has fallen from 48 to 31 percent, and the share of conservatives has dropped from 25 to 13 percent.

A 2014 Pew study showed a steady process of ideological consolidation among both Republicans and Democrats since 1994, although consolidation accelerated most rapidly on the left:

The share of Democrats who are liberal on all or most value dimensions has nearly doubled from just 30 percent in 1994 to 56 percent today. The share who are consistently liberal has quadrupled from just 5 percent to 23 percent over the past 20 years.

In a reflection of their importance in primaries, “consistently liberal” Democrats turned out in elections at a 70 percent rate, compared with 47 percent for “mostly liberal” Democrats and 41 percent for Democrats with “mixed views,” according to Pew data.

These and other trends, according to Lee Drutman, a senior fellow at the liberal New America think tank, represent the changing the mind-set of the Democratic electorate. In an email, Drutman wrote:

Two related dynamics are operative. One, Democratic voters have moved to the left in response to Trump, and are eager for policies that signal transformative change, not just incrementalism. There is no return to normalcy, because the past is now seen as more flawed.

This dynamic has intensified because the most engaged Democrats, Drutman writes,

have bought into an argument that Hillary Clinton lost in 2016 because she was too incrementalist, and had she gone a little bolder and promised something more exciting than her competence and experience, she might have won.

The second dynamic, Drutman continued, is that

in a crowded field, candidates are seeking to distinguish themselves with grass-roots energy. A very effective way of generating grass-roots energy is attaching to transformative changes these policies signal. Incrementalism and moderation doesn’t generate much excitement among the activists or in coverage from journalists who are responsible for driving candidate buzz in these early stages.

In a similar analysis, John Sides, a political scientist at George Washington University, argues that

On race and immigration in particular, I think Trump is pushing Democrats to the left. To be clear, Hillary Clinton was already to Obama’s left even in 2016. But I think Trump’s rhetoric and agenda has made conservative positions on race and immigration even more anathema to Democrats.

Sides also pointed out that the incentives for Democratic candidates have shifted:

It used to be the case that Democratic candidates had to moderate on racial issues to maintain a Democratic coalition that included both blacks and some racially conservative whites. But the Democratic coalition has changed: it is increasingly made up of not only nonwhites but whites who espouse racially liberal attitudes.

In an argument elaborating on a point made by Drutman, Edward Carmines, a political scientist at Indiana University, made the case that the 2016 defeat of Hillary Clinton fostered the shift to the left:

If a moderate policy agenda cannot guarantee electoral victory why downplay your leftward policy orientations in the unlikely event that it will jeopardize your electoral success?

Carmines added:

Trump’s success has reinforced this perspective. He won with a very right-wing policy agenda. Perhaps a left-wing set of proposals can lead Democrats to similar success.

Carmines believes that “both of these premises are mistaken and could very well lead Democrats to electoral defeat when victory is within sight.”

Leonie Huddy, a political scientist at Stony Brook University, argues that the liberal tilt of the Democratic candidates is, in fact, a rational political strategy. “There is real change afoot with the most recent generations,” she wrote, citing a Pew study released in January:

Young Republicans are especially different from older Republicans and the Democratic Party is likely to regard such findings as an invitation to poach more liberal young Republicans.

Strong partisans, however, Huddy writes, “are unlikely to defect from the party in 2020 regardless of its issue agenda,” and political independents