Impeachment has vividly captured this polarization. To a much greater degree than Democrats during Bill Clinton’s impeachment, congressional Republicans have locked arms around their president. In the House Intelligence and Judiciary Committees, nearly every Republican defended Trump or tried to discredit the witnesses against him, rather than trying to independently assess the underlying facts of the controversy. Nor have Republicans in either chamber expressed any resistance to Trump’s systematic defiance of Congress’s oversight requests and his incendiary accusations that treasonous Democrats are staging a coup.

House Democrats showed much more commitment to exploring the evidence and grounding their case against Trump in the findings of the House’s investigation. But they, too, supported their party on every key vote, swatting away GOP charges that the process was unfair. An effort by some moderates to explore censure as an alternative to impeachment quickly fizzled. And despite much uncertainty about the intentions of the 31 House Democrats in swing districts that voted for Trump in 2016, all but two of them voted last night to impeach him.

Read: Trump’s scarlet letter

The partisan unanimity on all these questions offers yet more evidence that 2020 could see historic turnout from each party’s core voters. Some experts have predicted that 2020 could produce the highest participation among eligible voters since the 1908 presidential election, before women had the right to vote.

In this “Battle of the Bulge” between two divergent Americas, small differences in both parties’ ability to mobilize supporters may tip the outcome. But in such an evenly matched conflict, small slivers of erosion at the edges of each coalition—or minor shifts in the opinions of any remaining swing voters—could also prove decisive. Recent trends in public opinion offer hints at the competing forces that will weigh on the ambivalent voters who could decide next year’s result.

In several polls, Trump’s approval rating edged up slightly during the impeachment struggle, though it remained at 45 percent or below in almost all surveys. Those numbers may reflect unease about the unprecedented prospect of Congress removing a president from office: Clinton’s approval rating also rose during the deliberations over his impeachment (though Richard Nixon’s fell steadily throughout the Watergate investigation).

It’s also possible that Trump’s approval has edged up for a more traditional reason: More Americans are expressing satisfaction with the economy. This fall, as the stock market ascended and the unemployment rate plummeted to 3.5 percent, Quinnipiac University national polls found that the share of Americans describing the economy as “excellent” or “good” rose from 59 percent in September to 73 percent this month, according to its latest survey.