Americans’ political behavior and beliefs have grown ever more partisan over the past 40 years. Democrats and Republicans alike have become more likely to support their own party’s candidates, to adopt their own party’s issue positions, and even to distort their perceptions of objective facts to fit their own party’s preferred version of reality. While political scientists have spent two decades documenting these trends, Donald Trump’s presidency has broadened and accelerated this process.

Republicans and Democrats’ attitudes toward politicians and political organizations are getting farther apart

To understand these changes, I compared the results of surveys conducted by the Internet survey firm YouGov in November 2017 and January 2020. The data were matched and weighted to be demographically representative of the adult U.S. population. The 2017 survey included 736 Republicans (including partisans and Republican-leaning independents) and 930 Democrats; the 2020 survey included 1,098 Republicans and 1,386 Democrats.

In 2017, Republicans and Democrats differed in their average ratings of President Trump by 5.8 points on a 10-point scale. By this January, the difference had grown significantly, to 6.7 points. The endpoints of the scale were labeled “extremely unfavorable feelings” and “extremely favorable feelings.” The share of Democrats who gave Trump a zero increased from 71 percent to 81 percent, while the share of Republicans who gave him a 10 increased from 28 percent to 48 percent.

Democrats and Republicans also grew farther apart on a wide range of other political attitudes and opinions. The partisan difference in average ratings of House Speaker Nancy Pelosi increased from 4.4 points to 5.8 points on the 10-point scale; of journalists, from 3 points to 4. The difference in views about regulating the environment increased from 2.3 points to 3.3 points. In these and other areas, the partisan disagreement increased substantially in just a little more than two years.

A rare exception to this pattern comes with attitudes toward Utah Sen. Mitt Romney, one of the very few current Republican officeholders to publicly criticize Trump (though his vote to impeach was not yet cast when the January 2020 survey was in the field). As Romney strayed from the party line, his average rating among Republicans nationwide sank from 4.8 in 2017 to 3.3 in 2020. But among Democrats, his average rating slipped from 3.5 to 3.3. Romney’s fate suggests that the only alternative to partisan polarization in the current political environment is to be equally disliked by both parties.

Republican Senate leader Mitch McConnell has taken the opposite course, revealing a clear example of partisan polarization in action. In 2017, views about McConnell were not very polarized, because even most Republicans viewed him unfavorably. But two years of conspicuous loyalty to the president changed that dramatically. His average rating among Republicans jumped from 4.0 to 5.4 on the 10-point scale, while his rating among Democrats dropped from 2.6 to 1.8, more than doubling the partisan difference from 1.4 points to 3.5.

The two figures locked in a battle for the Democratic presidential nomination at the time of the January 2020 survey, Joe Biden and Bernie Sanders, suffered similarly and about equally from intensifying opposition among Republicans and slippage among some Democrats, as well, resulting in only modest increases in partisan polarization. As presumptive nominee, Biden will now aim to improve his standing among Democrats without alienating Republicans.

We increasingly disagree about not just our leaders, but each other, as well. Views about whether “most Republicans” and “most Democrats” are moral, generous, knowledgeable and hard-working are significantly more polarized in 2020 than they were in 2017. So are feelings toward many other social groups that people often associate with one party or the other. Of 24 groups included in both surveys, the partisan gap in ratings widened for 21. The differences in average ratings of Muslims, Christians and wealthy people all increased by more than half a point on the 10-point scale. Perhaps most worryingly, the partisan difference in ratings of journalists, who are supposed to be the neutral observers of partisan politics, increased by nearly a full point.

Gaps in social attitudes and beliefs increased, as well

On issues on which Democrats and Republicans were already far apart in 2017 — such as health care and redistributing income — the gaps only increased slightly. But on other issues, especially those that Trump has emphasized, such as immigration, the partisan differences increased substantially. For instance, on preventing illegal immigration, the gap widened by 50 percent — largely because Democrats grew more strongly opposed to the administration’s actions. On regulating pollution and environmental hazards, the gap increased by 40 percent, as Republican support increased for Trump’s efforts to curtail environmental regulations. These increasing gaps illustrate how partisan loyalties shape the social and political attitudes we often point to as reasons for preferring one party or the other.

The power of partisan identities is underlined by the stability of Americans’ partisan allegiances in recent years. Because the respondents in both YouGov surveys were originally interviewed in the summer of 2016, we can track their partisanship through the fall 2016 campaign and much of Trump’s presidency. All the partisan gaps reported here are based on 2016 partisan attachments, so the widening differences between Democrats and Republicans reflect real shifts in attitudes, not just more efficient sorting of people with different views into opposing partisan camps.

Almost 92 percent of those who identified with or even leaned toward the Democratic Party in summer 2016 still did so in January 2020, with just 3 percent shifting or leaning toward the Republican Party. Conversely, 86 percent of 2016 Republicans were still with the party in 2020, while less than 5 percent now called themselves Democrats.

Will the pandemic bring Americans back together?

Some observers have suggested that the coronavirus pandemic could break our sharp polarization, uniting us in a shared sense of national emergency. That certainly hasn’t happened yet. Indeed, Democrats and Republicans disagree strongly about whether the coronavirus is even a serious threat, suggesting that our deepening political divisions will not begin to heal anytime soon.

Larry M. Bartels is a political scientist at Vanderbilt University. His books include “Democracy for Realists: Why Elections Do Not Produce Responsive Government,” with Christopher H. Achen (Princeton University Press, 2017) and “Unequal Democracy: The Political Economy of the New Gilded Age,” Second Edition (Princeton University Press, 2018).