OPINION — While President Trump complains about the national media, Democrats, Robert S. Mueller’s Russian “witch hunt” and the political establishment, none of those things is why the November House elections are a major headache for the Republican Party. Donald Trump’s biggest problem is Donald Trump.

Trump has turned what could have been a challenging midterm election environment into a potentially disastrous one. Through his tweets and statements, the president continues to make the 2018 midterm elections a referendum on his first two years in office.

Of course, that could be a good thing, since unemployment is down, economic growth is up and ISIS is in retreat.

But instead of running on those accomplishments, Trump prefers to stir the pot of grievance, drawing applause from his hard-core supporters for his attacks on individuals and institutions, and refusing to reach out to potential new supporters.

He goes after Republican officeholders, professional basketball stars, NFL players and members of Congress.

He whips up anger toward the media, undermines the FBI and criticizes America’s allies and NATO.

He imposes tariffs that hurt American agriculture.

That might not be a terrible strategy if Trump had won comfortably in 2016. But he lost the popular vote by more than 2 points and drew only 46.1 percent of the vote, so any leakage from his original coalition or increased turnout from anti-Trumpers could have a dramatic impact on the midterm’s results.

In fact, Republican self-identification seems to be slipping, while Democratic enthusiasm is up.

Typically, when a president is unpopular, candidates from the president’s party try to “localize” their races. They want voters to focus on the individual nominees — their records and qualifications — rather than on the performance of the president.

But that is difficult to accomplish when the president dominates the news and makes controversial comments daily.

Trump clearly loves rallies. As an entertainer, he enjoys (even craves) being the center of attention. He is energized by the applause and cheers. His success in swaying GOP primaries through his endorsements has also fed his ego, which in turn has increased his desire to do more events and to whip up his audiences with more and more outrageous assertions and charges.

Not surprisingly, the president recently promised that he will be on the stump almost continually for Republican nominees in the fall.

“I’ll go six or seven days a week when we’re 60 days out, and I will be campaigning for all of these great people that do have a difficult race, and we think we are going to bring them over the line,” he said during a recent interview with Sean Hannity.

That strategy may feed the president’s ego and reflect his view that he is his party’s best advocate, but it shows he misunderstands the midterm dynamic.

Trump’s national campaign blitz will no doubt generate effusive applause in Mississippi, rural West Virginia and northeastern Pennsylvania, but it is not helpful in suburban counties with college-educated voters, congressional districts carried by Hillary Clinton in 2016 or even competitive Republican-leaning congressional districts.

It isn’t helpful for GOP Reps. Barbara Comstock of Virginia, Leonard Lance of New Jersey, Jason Lewis of Minnesota or Rod Blum of Iowa.

Trump’s campaign plan guarantees the November midterms will be a referendum on the president — not the “local” contests so many Republican nominees in swing districts prefer.

That could help Republican Senate nominees in Missouri, Indiana, North Dakota and West Virginia, but it increases the likelihood that the House will flip to the Democrats.

To be sure, given the president’s performance during his first two years in office, the Democrats were always going to make Trump the issue in the midterms. But by being so divisive and so active on the stump, the president has made it easier for the Democrats to nationalize the November elections and more difficult for those Republicans who are trying to swim against the midterm tide.

Because Trump thinks that everything is about him, he is simply incapable of receding into the background or allowing the midterms to be about anyone other than himself. And because he cannot acknowledge his own missteps and relies on caricatures and exaggeration to demonize his foes, he is incapable of reaching out to voters who are not already reliable members of the Trump base.

The combination of those flaws makes the president of the United States the biggest problem for the Republican Party this year. Donald Trump has met the enemy — and it is himself.