... My book defending the scale and durability of Obama’s legacy came out during the peak moment of fear (from the left) and confidence (from the right) in Trump’s powers. The alleged ease and certainty with which Trump would reverse Obama’s achievements was invoked by critics from the right (“Jonathan Chait’s Audacity, which has the unfortunate distinction of having gone on sale 72 hours before Donald J. Trump took the oath of office, rendering it utterly irrelevant as anything but a cultural artifact demonstrating the hubris of American liberalism”) and the far left (“Most of the successes Audacity touts will likely be obsolete in the next few months. Congressional Republicans have already begun the process of repealing the Affordable Care Act.”)

As I explain in that book, this was a strange and flawed way to think about the Obama legacy. Large chunks of Obama’s achievements are not even theoretically vulnerable to reversal — most obviously, the stimulus, bank rescue, and auto bailout, which rescued the economy from a second Great Depression. Trump would desperately love to repeal the Dodd-Frank law, which restructured the finance industry and established the Consumer Financial Protection Bureau, but his party lacks the filibuster-proof supermajority required to attempt it.

What’s more, the practice of judging a president’s record by his successor’s antagonism to it is a new one, seemingly invented for Obama. Andrew Johnson undermined Lincoln’s legacy of liberating the slaves; conservative Republicans have tried to privatize or cut programs like Social Security and Medicaid decades later. Large chunks of the regulatory state — like the National Labor Relations Board or the Securities and Exchange Commission — now function only intermittently, turning feeble when Republicans control the Executive branch. And yet these facts do not usually make historians dismiss the importance of abolition or the New Deal or the Great Society.

Trump’s presidency poses a threat to many enduring institutions of government, including the Constitution itself. That stark reality is no more reason to dismiss Obama’s legacy than to dismiss James Madison’s. And as the scale and nature of Trump’s strange mix of fanaticism, corruption and incompetence sets in, we are coming to see it as a thing of its own, not the negation of the administration that preceded it. ...