Memo to Washington: We better resolve our differences with Mexico before November. An intense presidential campaign south of the border only occasionally involved the hot immigration and border issues dominating our politics — and when Mexicans vote Sunday, they’ll mostly consider hot issues at home: corruption, crime, and the economy.

Yet by December 1, when a new president is sworn in in Mexico City, it may be too late for us to resolve an immigration debate that, regrettably, our politicians would rather keep as an ever-simmering wedge issue.

Democrats and Republicans don’t seem to be in any rush to pass legislation resolving what they deem a border “crisis.” Another bill failed in the House on Wednesday. For now, the issue is motivating voters on both sides gearing up for the midterms.

If they think the issue will be any easier to solve in the fall, they should think again. When it comes to immigration and the border, presidential candidates in Mexico have been trying to outdo each other in showing how tough they’ll be toward President Trump.

Trump wears his disdain for Mexico on his sleeve and therefore is as uniformly despised by Mexicans as the national soccer team is loved. (A poor showing against Sweden Wednesday notwithstanding.)

Take the outgoing president, Enrique Pena Nieto. He was accused by some would-be successors of giving aid and comfort to the enemy by agreeing to host Trump during the 2016 campaign. Now on the anti-Trump bandwagon, he vows Mexico will “never” finance Trump’s hated wall.

Or Ricardo Anaya, a maverick full of fresh ideas leading a left-right coalition. When immigration finally became an issue in one of the campaign’s three televised debates, Mr. Anaya said Mrl Pena Nieto’s much-maligned 2016 meet with Mr. Trump was nothing less than a “humiliation” for the Mexican people. Of Mr. Trump, he said, “You can’t appease tyrants and bullies. You have to confront them.”

Yet, tough talk aside, it’d be difficult for a Mexican leader to wage political war against America. Our two countries are too intertwined.

As for immigration, more Mexicans are returning home these days than migrating north. Border crossers are largely from Central American countries, fleeing socioeconomic conditions far worse than Mexico’s.

So if Mr. Anaya wins (or, as unlikely as it may be, Jose Antonio Meade, the candidate of PRI, Pena Nieto’s party), he’ll likely keep talking tough at Mr. Trump. Government-to-government relations between Mexico and America, though, will remain close and — yes — beneficial for both sides.

Watch out if Andre Manuel Lopez Obrador, widely considered the most likely winner Sunday, gets in.

“With the triumph of our movement, we’ll defend migrants from Mexico, Central America and the whole continent and all migrants from around the world who need to leave their towns to go and make their life in the US,” AMLO (as he’s known) told supporters at a recent campaign stop, according to the Financial Times.

Struggling to shed his true colors as a Hugo Chavez-like populist, AMLO vowed that once he assumes power Mr. Trump is “going to have to learn to respect us.” Yet, he also promises a mano-a-mano sit-down with Mr. Trump to renegotiate border and other issues.

With Mr. Trump’s love for summitry and both sides cocksure of their deal-making prowess, what could go wrong?

In reality, AMLO is allied with Latin America’s darkest regimes. To strengthen his hand while negotiating with Mr. Trump, he may well encourage allies in Venezuela, Nicaragua, and other Bolivarian revolutionaries to send their worst to flood our border.

Rather than coordinate with American colleagues on immigration, crime, and border control, Mexican officials under AMLO will cultivate relations with America’s regional (and global) rivals. With his natural allies in the region, AMLO will widen the scope and the stakes of the border crisis.

So we best get our act together before AMLO enters Los Pinos, Mexico City’s seat of power, which is a real possibility. By next winter, what now seems like the hottest of all political hot potatoes will be much tougher to resolve.

This column first appeared in the New York Post.