Despite a midterm election that showed a significant ­decline in support for the GOP in America’s suburbs and no corresponding increase in support elsewhere, Donald Trump has chosen to let Donald Trump be Donald Trump. In fact, he’s Trumpier now — in policy terms — than ever before.

He is pulling US forces out of Syria. He has intimated that he would redeploy our troops out of Afghanistan, as well. He has gotten his trade war with China. And he has shut down the government to get funding for his wall.

These policy choices are all congruent with the ideology he has espoused since he began making public pronouncements on politics in the 1980s.

So why, if Trump is keeping his campaign promises, is he ending the year in such a mess, getting no credit for holding fast except from people who will apparently give him credit for anything and everything?

The results of the midterm would have suggested to a more conventional politician that he needs to modify his approach, to get back the suburbanites he has lost at the very least, and the easiest path would have been to demonstrate his effectiveness as a leader.

But that isn’t what we’re going to see. Trump isn’t looking to cut deals with Democrats. Indeed, when Vice President Mike Pence did so on a budget deal, Trump cut him off at the knees.

Trump is digging in. And maybe this is what the country both needs and deserves. He ­became our president and then found himself restrained for various reasons when it came to pursuing his more disruptive aims. In part this was due to the fallout from his earliest disruptive effort, the seven-nation Muslim ban.

The move was incompetently devised and created chaos, not a feeling of safety. Politically, it was self-defeating, since it galvanized the #Resistance in much the way President Barack Obama’s stimulus helped organize the Tea Party in 2009.

At the same time, Trump was unable to take advantage of the disruptions his own party ­offered to him on a silver platter. His first policy failure was ­insisting that ObamaCare be ­repealed and replaced at the same moment, rather than ­allowing the GOP to repeal it with a two-year implementation period for its replacement.

Had he gone with the repeal-now-replace-later approach, he would have scored an immediate early victory. It would have been disruptive and effective. Instead, his ignorance of the workings of Congress led to a year of bad headlines, unsuccessful votes and GOP divisions.

Also after the 2016 election, Speaker of the House Paul Ryan took Trump both seriously and literally and came up with a proposal for a “border adjustment tax” to fund the border barrier to the tune of $60 billion. It was a complicated system of money collection used by 60 other countries that would have ­allowed Trump to claim he was making Mexico pay for the wall.

Instead, he wasted time on disrupting the NFL and disrupting the caravans and picking fights with widows. There is no question that many people loved these bits of culture war, and he did rally his base. Republican turnout in the midterms was overall very high.

Alas for the Republicans, Democratic midterm turnout was astronomical, only a few million ballots short of the number cast for Hillary Clinton two years earlier. Trump’s incessant culture-war disruptions alienated suburbanites. Thus, they were very much part of the reason he may well find himself in the crosshairs of an impeachment bazooka soon.

But maybe this is all for the good of the nation. A real Trumpian policy shift will force a ­national discussion on the things that really matter, not least foreign policy.

After all, we can’t say we weren’t warned about the neo-isolationism he’s apparently getting ready to let loose. He was restrained from implementing it over the past two years. But no longer.

If foreign policy becomes a major factor in the 2020 race, that will be all to the good. There is no longer a coherent national or even intra-party consensus on these matters, and an election will help to clarify them for good or ill.

Yes, I’m worried the clarification will be for ill — for retracting inside an American bubble. But the country can’t sustain a more internationalist foreign policy if the public is no longer interested in one.

Maybe Trump is retreating into a comfort zone, or maybe he is liberated because he feels like he has nothing to lose. Either way, he is going to force the country to face desperately ­important questions he — and his enemies — have too frequently sought to distract us from.