The third one stands to remake our constitutional democracy and undermine the capacity for future change. The first two problems will become much harder to address as time goes on. Three areas-climate change, the risk of a renewed global arms race, and control of the Supreme Court-illustrate the historic significance of the 2020 election.

The effects of a full eight years of Trump will be much more difficult, if not impossible, to undo. Whatever you think of these actions, many of them can probably be offset or entirely undone in the future. After four years as president, Trump will have made at least two Supreme Court appointments, signed into law tax cuts, and rolled back federal regulation of the environment and the economy. More countries may opt to pursue nuclear weapons, especially those in regions that have relied upon American security guarantees. But some situations are not like that a zig one way makes it hard to zag back. The contending parties take our system’s rules as a given, and fight over what they understand to be reversible policies and power arrangements. In normal politics, the policies adopted by a president and Congress may zig one way, and those of the next president and Congress may zag the other. But even if we take those off the table-even if we assume that Trump continues to be hemmed in by other parts of the government and by outside institutions, and that he governs no more effectively than he has until now-the impact of a second term would be more lasting than that of the first. So far, much of the concern about the long-term effects of Trump’s presidency has centered on his antidemocratic tendencies.

Alan Abramowitz, a political scientist at Emory University who weights both of those factors heavily in his election-forecasting model, gives Trump close to an even chance of reelection, based on a projected 2 percent GDP growth rate for the first half of 2020. The other is a relatively strong economy (at least as of now). One is incumbency: Since 1980, voters have only once denied an incumbent a second term. But despite the scandals and chaos of his presidency, and despite his party’s midterm losses, he approaches 2020 with two factors in his favor. Trump’s reelection seems implausible to many people, as implausible as his election did before November 2016.

Of all the questions that will be answered by the 2020 election, one matters above the others: Is Trumpism a temporary aberration or a long-term phenomenon? Put another way: Will the changes brought about by Donald Trump and today’s Republican Party fade away, or will they become entrenched?

His Third, Her Second by Paul Estaver