To the extent that this mix of attitudes is a defining feature of Trumpism, and Trumpism is taking over the Republican Party, this presents a serious challenge to American democracy. Many Americans identify as Republicans, and most of the time, voters look to their party leaders to tell them what they should think about the issues.

Thanks to strong support from conservative media and explicit or implicit support from Republicans in Congress, Mr. Trump is redefining what it means to be a Republican. We don’t know yet how much his leadership will erode support for democracy among his loyal base, but without powerful conservative voices offering an alternative vision of what it means to be a Republican, it’s unlikely that most Trump voters will push back on his illiberal impulses on their own.

We worry what might happen if Mr. Trump stokes well-honed Republican concerns about voter fraud as the November midterm elections approach. If Republicans lose the House or the Senate based on a few close contests, will Mr. Trump challenge the results and encourage his partisan supporters to do the same? At the very least, Mr. Trump’s continued anti-system, illiberal appeals, which surface whenever he campaigns, continue to energize his base (his recent performance in Pennsylvania is a good example).

The highly polarized state of our two-party system accentuates the danger. If Mr. Trump transgresses more seriously against democratic norms, many Republicans might go along with him simply out of distrust of or enmity toward the Democrats. And many others might simply disengage from politics, unhappy with either alternative and deeply frustrated with the whole political system. This creates its own set of problems, since we also find (like many before us) that those who are most disengaged from politics are no great lovers of democracy.

Still, to return to the good news for a moment: Public support for democracy as a “good” or “very good” system has returned to a historic high in the past two decades (86 percent), as has the proportion (82 percent) attaching great importance to living in a democracy.

We don’t take our good news as cause for comfort. The lesson we draw from our data — and from the current global democratic recession — is that democracy is not self-sustaining. It depends on public commitment to its essential values: pluralism, mutual respect and tolerance, flexibility, critical thinking and a willingness to compromise.

These must be renewed with every generation through civic education. And they must be modeled and defended by political leaders and other elites on all sides, who have always been the most important teachers and sustainers of democracy.