It’s this combination of factors that makes American politics so uniquely volatile at this moment. The country is deeply divided between two equally matched coalitions: Neither side has been able to establish a durable advantage over the other for the past half century. Since 1968, one party has simultaneously controlled the White House, the House, and the Senate for only 14 years. The past four times a president went into a midterm election with unified control of government, most recently Trump in 2018, voters revoked it. Neither Democrats nor Republicans can truly be confident about the outcome of the presidential race in 2020, and while each party might be favored to hold the congressional chamber it now controls, neither advantage is impregnable.

Read: Why Trump’s favorite 2016 map should scare him

The long-term demographic trends in the electorate—more racial diversity, more college graduates, more urbanization, more voters who aren’t Christian—benefit Democrats, but those advantages are offset by signs that those very changes are leading more white voters wary about them to back Trump. Republicans think they can squeeze out larger margins from shrinking groups; as a long-term strategy, that’s a dicey proposition. But it could prevail in the near term, especially since both the Electoral College and the Senate benefit small states that remain mostly white and Christian. Amid such closely balanced contending forces, both parties live in constant fear that even the tiniest of blunders will lead to victory for the other.

That the parties are growing in their differences only compounds that fear. Election outcomes now produce whiplash-inducing reversals in policy outcomes, since the two sides represent coalitions with such divergent priorities and preferences. Polling by the nonpartisan Public Religion Research Institute captured that separation: In an October survey, 92 percent of voters who approved of Trump said Republicans are working “to protect the American way of life against outside threats,” while 75 percent of voters who disapproved of him said the GOP has been taken over by racists. Conversely, three-fourths of Trump approvers said Democrats have been taken over by socialists, while three-fourths of those who disapproved of him said the Democratic Party is endeavoring to make capitalism work better for average Americans.

Separate polling from Pew has found that Democrats and Republicans hold views of the other that are growing more negative, with GOP partisans especially likely to view Democrats as immoral and unpatriotic. While most Democrats in the Pew poll indicated that they would prefer their party to seek common ground with the other, most Republicans did not—attitudes that explain both the appeal to Democratic voters of former Vice President Joe Biden’s promise to seek bipartisan cooperation if elected and the widespread skepticism among leaders in both parties that he’s likely to obtain it.