It sounds obvious, but apparently not obvious enough to President Trump: Don't insult an entire state. Especially one that's critical to you and your party's future election hopes.

But just six days after getting inaugurated, Trump went there.

“I won New Hampshire because New Hampshire is a drug-infested den,” Trump told Mexican President Enrique Peña Nieto in a phone call between the two world leaders. The call was private, but The Post's Greg Miller obtained the transcript — plus an equally eyebrow-raising one of a call with Australian Prime Minister Malcolm Turnbull.

Back to Trump casually depicting an entire state as one big heroin den. There are quite a few things that could come back to haunt Trump politically based on that one sentence.

1. New Hampshire is a must-win primary state

Trump was explaining to Peña Nieto that “drug lords in Mexico are knocking the hell out of our country.”

It's classic Trump, the exact same person as the one who descended the escalator of Trump Tower more than two years ago and declared Mexican immigrants are “bringing drugs. They’re bringing crime. They’re rapists.”

Framing the country's economic and social problems in the context of a boogeyman drug trade arguably helped Trump win the Republican presidential nomination and, ultimately, the presidency. New Hampshire was his first primary win (after coming in second in Iowa), and, in hindsight, it was the moment Trump became a real contender. To get there, he talked about drugs. A lot.

“This is going to be a real wall,” Trump promised at a February 2016 rally in Manchester, days before he'd win the Republican primary there. “This is going to stop the heroin and the drugs from coming to New Hampshire.”

Could he be accused of simplifying New Hampshire's opioid epidemic? Maybe. But no one accused Trump of directly insulting the people of New Hampshire. Until now.

“It’s disappointing his mischaracterization of this epidemic ignores the great things this state has to offer,” said the Republican governor of New Hampshire, Chris Sununu, a Trump ally.

2. New Hampshire would be critical to Trump reelection

If Trump wants to run again, it would help to have New Hampshire on his side. If a Republican challenges him in the primary, New Hampshire will be a must-win. Imagine the narrative of a sitting president losing the second state to cast its vote.

New Hampshire is a swing state in the general election. Trump proved he can win without it — Clinton won New Hampshire by less than half a percentage point. But if you're trying to put together an electoral map win, New Hampshire is a state you don't want to discount, much less upset.

One of the best ways to determine something's political impact is to imagine it in a TV ad. “President Trump thinks your state is a drug-infested den.” Yeah, ouch.

3. He just ticked off two powerful Democratic senators

More than six months into his presidency, Trump has no legislative wins besides getting his pick on the Supreme Court, and no working relationship with Congress.

One promising area (irony alert): Legislation combating the opioid epidemic.

Last year, Congress passed a bipartisan bill creating federal grand and programs related to helping the more than 2 million Americans estimated to be addicted to heroin and opioids. New Hampshire has been hit particularly hard: The state ranks in the top two in 2016 for opioid deaths per capita.

But Trump did not give himself any leverage in that debate. The two New Hampshire senators took immediate offense:

4. It demonstrates a complete misunderstanding of one of his central campaign issues

If Trump's appeal was rooted in his commitment to combating drug abuse, he just showed a remarkable lack of understanding of the problem.

While talking to the Mexican president, Trump appears to be blaming Mexican cartels for New Hampshire residents' drug addiction. But the opioid crisis has its roots in prescription drug use, specifically pain killers like Oxycotin and Fentanyl, which can come from your local doctor or as far away as a lab in China. Trump's own opioid commission (more on that later) cited prescriptions as one of America's biggest drug addictions: “In 2015, the amount of opioids prescribed in the U.S. was enough for every American to be medicated around the clock for three weeks,” they wrote in a recent report.

Wonkblog's Chris Ingraham has more specifics about New Hampshire's opioid problem and how it compares to other states. Heroin, some of which comes from Mexico, is part of the equation. But it's far from the only opiate killing Americans.

5. It raises the bar on expectations for Trump to fix the problem, big league

If Trump thinks the drug epidemic is so bad that an entire state is a “drug-infested den,” then why isn't he doing something about it?

His own commission on the opioid crisis, led by New Jersey Gov. Chris Christie, released a report Monday requesting urgent action by the president: declare the opioid epidemic a national emergency. That would open up federal funds and create a national focus on the crisis, which they went so far as to compare to America's deadliest terrorist attack.

“With approximately 142 Americans dying every day, America is enduring a death toll equal to September 11th every three weeks,” the report reads.

Trump has yet to announce any policy solutions, but actually working to fix its opioid problem might be the best way to help New Hampshire forget about his insult.