Isaac Newton’s third law of motion states that for every action in nature, there is an equal and opposite reaction. It can operate in politics, too. For example, Harvard law professor Jack Goldsmith recently wrote, “It is part of Trump’s evil genius that he elevates himself by inducing his critics to behave like him.”

Call it Trump derangement syndrome, and recognize it for what it is: something that could end up snatching defeat from the jaws of victory for the Democratic Party once again in 2018 and 2020.

Signs of that possibility are apparent in the polls. President Donald Trump’s job approval has remained low, by historical standards, but it has also remained pretty steady — and has been rising, just a bit, in recent weeks.

The standard pattern has been for presidents to start off their term with high honeymoon ratings and then sag somewhat in their second year unless buoyed (as both Bushes were) by perceived foreign policy successes.

What we saw in the midterm elections of 1994, 1998, 2006, 2010 and 2014 was the opposition party’s winning majorities in both houses of Congress, except for the Senate in 2010.

Trump’s trajectory has been different. After his controversial campaign, his barbed tweets and the revelation of the Hillary Clinton campaign-financed Steele dossier, he was never in honeymoon territory. His average job approval rating in the RealClearPolitics average never topped 46 percent — the same percentage he won in the popular vote against Clinton.

In succeeding months, his numbers oscillated within a narrow range and often with no discernible (to the press, anyway) connection with events. As in the 2016 campaign, stories that would have hurt other politicians (Stormy Daniels, anyone?) seemed to have been priced in on attitudes toward Trump. You loved him or hated him — and kept on doing so.

His recent upswing has his approval at 43.5 percent — well below 50 percent but far higher than the 35 percent George W. Bush had before the Republicans’ thumping in 2006.

Perhaps this reflects the economic upswing since the Republican tax bill passed in December. Perhaps it reflects presidential initiatives on Korea, Iran and China or the respect shown to him by the leaders of France, Saudi Arabia, Japan, Germany and others. He may be uncouth, some may think, but he’s getting results.

And perhaps it reflects the Democrats’ Trump derangement syndrome.

You had the spectacle of Bernie Sanders and 41 Democrats, including every Democrat on the Senate Foreign Relations Committee, on the Senate floor opposing Trump’s nominee for secretary of state — a nominee, Mike Pompeo, whose confirmation as CIA director some of them had voted for and who had been getting good marks at Langley.

Looking ahead, it’s possible that Republicans in 2018 and Donald Trump in 2020 could win based on solid achievements. But their chances will be aided if Democrats can’t shake off their bad case of Trump derangement syndrome.

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Michael Barone is a senior political analyst

for the Washington Examiner.