President Obama’s legacy is rapidly vanishing. The decision by President Trump to withdraw from the nuclear deal with Iran is the biggest blow, but it’s only the latest. The elimination of the individual mandate and canceling the yearly bailout of insurance companies have left Obamacare in a precarious condition. Young immigrants whose parents brought them to the United States unlawfully—so-called dreamers—are losing their legal status.

This is historic. Presidents often vow to wipe out big chunks of their predecessor’s legacies. President Eisenhower was going to take on the New Deal. Ronald Reagan targeted the Great Society. Both backed down. Trump, working with congressional Republicans, hasn’t. He’s eager to deflate Obama’s standing and inflate his own.

Obama and Democrats have made Trump’s efforts surprisingly easy. Obama, you’ll recall, succeeded brilliantly in the first two years of his presidency when Democrats controlled Congress. But once Republicans held the House, Senate, or both over the next six years, he ignored Capitol Hill as much as possible. He spared himself the unpleasantness of compromising with Republicans and instead governed by executive orders and regulations.

Decisions taken by the president alone are vulnerable to being erased by subsequent presidents. And that’s what happened to the pact with Iran. It wasn’t a treaty ratified by the Senate. Democrats used the filibuster to block even a nonbinding vote on it. Trump killed the deal with his signature. That was also all it took to quit the Paris accord on global warming.

There were two factors behind Obama’s decision to shun a treaty, which requires a two-thirds vote in the Senate. Winning that lopsided a vote appeared to be impossible. On the other hand, Obama had a backup—Hillary Clinton. She was expected to win the presidency in 2016 and could be relied on to protect the nuclear agreement.

On immigration, Obama might have prevailed if he’d sought congressional approval of legal status for young illegals, the dreamers. Again, Republican votes would have been needed, which meant the bill would be a compromise, not pure Barack. He rejected that. Since Obama had said he couldn’t legally act on his own, it looked like nothing would be done.

Then Obama changed his mind and simply announced the approach known as Deferred Action for Childhood Arrivals. Neither a regulation nor a law, it is simply a policy. With Obama gone, it lacks a presidential defender. Trump abandoned it, though he’s offered a pact to keep the dreamers here: They’d be legalized and Trump would get his wall on the southwest border. A pretty straightforward exchange, except Democrats oppose the wall. It’s more important to them than the fate of hundreds of thousands of dreamers. Democrats declined.

Which leads to another problem they’ve created for themselves, one that further jeopardizes Obama’s legacy while making political life less of a struggle for Trump and Republicans. Democrats have adopted a strategy of resisting Trump across the board. It’s blind resistance, all anger and ideology, no common sense.

And there haven’t been many exceptions. Perish the thought. When the GOP tax bill was under consideration last year, private talks—chats, really—between a few House Republicans and Democrats blossomed. Republicans were willing to discuss a smaller tax cut, if only to pick up Democratic votes and assure passage. But when a particular tax idea was put on the table, one Democrat’s response was, “If Trump’s for it, I’ve got to be against it.” That attitude left no room for any compromise. It also meant Democrats would have no influence on the tax bill.

I asked a Republican leader what Democrats might have gotten if they’d pitched in. Not only would the size of the tax cut have been trimmed, he told me, but Obamacare’s individual mandate and the full deduction for state and local taxes would have been preserved. It would have allowed Democrats to claim the tax bill “could have been worse” absent their intervention. They were right. From a Democratic standpoint, it’s much worse. That’s why Trump and most Republicans like it.

As much as Obama and Democrats are to blame, the shrinking of Obama’s legacy isn’t entirely their fault. A good bit is the result of Trump’s success and canny choice of issues. On foreign policy, does anyone want to return to the days of appeasing North Korea? Should the American embassy in Israel be moved back to Tel Aviv at the earliest opportunity? Ought we go back to insisting that concessions by Israel provide a path to peace in the Middle East?

On domestic issues, the Obama legacy has better prospects for survival, or at least for being revived by some future Democratic majority. The Democratic mindset on taxes is locked in place. Nancy Pelosi embodies it. She’s impervious to such things as incentives, private investment, and growth. She’s for raising taxes because cutting only benefits the rich. She’s already rich. One can make a case that she’s more influential on the tax issue than Obama.

But all they’ve said and done as Mr. and Mrs. Tax Hike hasn’t changed the country’s mind. It will be an uphill battle to convince Americans to go back to higher taxes. That’s not much of a legacy.

