The 2020 Democratic primary field has been touted as far more liberal than that of previous years. Candidates have proposed a number of progressive policies that were not even under consideration in the last presidential election, such as decriminalizing border crossings, levying higher taxes on the wealthy and offering reparations to descendants of enslaved men and women. What’s more, two of the field’s most liberal candidates, Sens. Elizabeth Warren and Bernie Sanders, still sit atop the national polls. Is this apparent leftward shift in the Democratic Party real? And if so, what’s driving it?

To answer this, we looked at data from the General Social Survey that tracks public opinion on the role of government in a variety of different policy areas between 1986 and 2018. And while that means we can’t track opinions on specific policies that have dominated the 2020 race, like Medicare for All, we can look at how public opinion more broadly has changed in the last 30 years.

First, the data shows that Democrats have indeed become more liberal over time, particularly on questions related to race and immigration. For instance, the share of Democrats who think the government has a special obligation to help improve black people’s standard of living due to past discrimination increased by over 20 percentage points between 1986 and 2018, while the share who think the number of immigrants to the U.S. should increase rose from 10 percent in 2004 to 35 percent in 2018. Democrats also moved to the left on health care, but as you can see in the charts below, Democrats have long been supportive of the government taking a more active role in health care, whereas support for issues of race and immigration have experienced a sharp uptick in recent years. (Of course, it’s not just Democrats moving to the left on these issues. There’s evidence that the general electorate is becoming more liberal, too.)

So who in the Democratic Party is driving this leftward shift we’re seeing?

Perhaps unsurprisingly, in recent years, the share of Democrats who identify as liberal and support more liberal positions is greater than the overall share of the Democratic Party who support those positions — meaning that liberals seem to be responsible for much of the broader leftward shift.

But this shift isn’t present in just the party’s more liberal members. We also saw movement from other demographic groups, depending on which question we looked at.

For instance, on the question of whether the government has a special obligation to help black people as a result of past discrimination, we saw a distinct gap in opinion when breaking down respondents by race. Far more black Democrats were in favor. White Democrats, on the other hand, have historically been less liberal on this question. The majority of the movement we saw in recent years was among white Democrats, who got closer to black Democrats on issues of race, as well as liberal Democrats, who broke away from conservative and moderate Democrats.

By contrast, on the question of whether the government should increase immigration to the U.S., we saw no racial divide. Instead, this movement seems to be driven largely by more educated Democrats and again, by liberal Democrats.

The first thing to understand about this leftward shift is that although there is evidence that more people — especially white people — are shifting parties based on how their views on race fit into the party, this change is not just driven by more conservative voters leaving the Democratic Party.

One way we know this is through research that has tracked public opinion among the same group of Democratic voters over time. Andrew Engelhardt, a postdoctoral research associate at Brown University, found in a forthcoming paper that while Democrats’ movement on race in the 1990s was largely driven by more socially conservative Democrats leaving the party, the Democrats surveyed in the 2000s were updating their opinions to become increasingly liberal.

And this trend has held up in other research as well. Dan Hopkins, a FiveThirtyEight contributor and professor of political science at the University of Pennsylvania who studies racial politics and political behavior, interviewed the same group of 500 Americans multiple times from 2007 to 2018 and found that racial prejudice has decreased among white Americans, particularly among white Democrats.

This means that the recent uptick in the share of voters holding liberal beliefs is driven not just by the departure of conservative voters, but also by Democrats themselves becoming more liberal. It is, of course, hard to identify just one reason the Democratic Party has shifted leftward in recent years, but in my conversations with experts, they all pointed to party elites (both politicians and influential liberal voices online). Now, some research has found that cues from the parties and party elites are even shaping voters’ personal beliefs, particularly on issues of race and immigration.

But why have so many Democrats moved to the left on these issues? On the one hand, the fact that race and immigration played such a central role in the 2016 election was certainly a contributing factor. A 2018 study by Peter Enns at Cornell University found that rather than voters choosing a candidate who matched their views on controversies like the Black Lives Matter movement, they actually changed their own views to match those of their preferred candidates. And there is evidence that Trump is continuing to drive some of this — although, perhaps not in the way one might expect. There isn’t evidence, for instance, that his rhetoric has contributed to an uptick in racist and sexist attitudes among white voters; instead, as FiveThirtyEight contributor Matt Grossmann has written, “the evidence shows that liberal-leaning voters moved away from [Trump’s] views faster than conservatives moved toward them.”

On the other hand, it’s also true that on questions of discrimination and immigration, there was plenty of room for Democrats to adopt more liberal stances. In our analysis, we found that as late as 2004, roughly the same share of Democrats and Republicans (10 percent and 8 percent, respectively) were in favor of increasing immigration. But by 2018, the share of Democrats in favor of increasing immigration had more than tripled to 35 percent, while the share of Republicans had only ticked up by four points. This marks a stark contrast from issues like economic redistribution, where the Democrats had already staked out pretty liberal stances. Thirty-nine percent of Democrats thought the government had a responsibility to reduce income differences in 1986. That number sits at 44 percent today.

But although the Democratic Party has moved to the left in recent years, a continued leftward trend is not inevitable. Some of the big, progressive ideas in the primary have been criticized for being too liberal. And while the share of liberals in the Democratic Party is certainly growing, 53 percent of Democrats still identify as moderate or conservative, according to data from Pew. It’s also important to keep in mind that some of the movement we’re seeing on race and immigration is a reaction to the Trump presidency, meaning we might expect it to wane moving forward (especially if he does not win reelection). At the same time, it’s hard to imagine Democrats making a dramatic departure on issues of discrimination and immigration, so if Democratic Party elites continue to direct voters’ attention toward these issues, Democratic voters may move even more to the left.