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.

That’s as unprecedented as Trump’s insulting tweets — and less fact-based than many. Some Democrats complained about Pompeo’s stances on gay issues. But they’re the party that blocked for seven months the nomination of a gay ambassador to Germany, Ric Grenell.

Another spectacle of Trump derangement syndrome was last Saturday’s White House Correspondents’ Association dinner, where a comedian’s vitriolic monologue and mean-spirited attacks on Trump’s press secretary validated his decisions this year and last not to attend. The event only further undermined the credibility of the anti-Trump press.

Its credibility may be further reduced if, as seems likely, special counsel Robert Mueller’s investigation ends with no finding of collusion between Russia and the Trump campaign. Much of the press, notably CNN, has treated the collusion story as a second Watergate, and many Democrats amuse their friends with little quips assuming Trump administration policy is set in Moscow. Not very funny anymore.

The collusion that seems more likely to have occurred is between Obama administration intelligence and law enforcement personnel and the news media to push the Russian collusion story largely or solely on the evidence of the Steele dossier.

Meanwhile, House Democratic leaders Nancy Pelosi and Steny Hoyer try to hush candidates baying for impeachment to meet the demands of party megadonor Tom Steyer and the majority of Democratic voters.

And they try to tilt local Democratic primaries toward candidates with military or law enforcement backgrounds and against “Resistance” types visibly afflicted with Trump derangement syndrome.

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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