OPINION — Imagine an alternative universe where Donald Trump understands words like “presidential” and “emotional self-control.” In that science fiction world, the 45th president’s Tuesday meeting with the Democratic congressional leadership would have served as a model for his entire administration.

In case you missed this rare moment of calm and comity, Trump put tantrums and tumult aside for 90 minutes to discuss infrastructure. The result: an agreement on the broad outlines of a $2 trillion plan to upgrade roads, bridges, airports, broadband and probably Pony Express relay stations.

Granted, this agreement in principle (not a word normally associated with Trump) is so airy that it risks flying off the printed page. From intense Republican skepticism about paying for it on Capitol Hill to pointed Democratic suggestions about funding it by rolling back part of the 2017 tax cuts, this deal probably has a shorter shelf life than a Trump nuclear accord with North Korea.

The funding questions have been conveniently saved for a late May meeting, which means that Tuesday was all cake and no calories. But in thinking big about infrastructure, Trump reportedly said, according to Democrats at the White House, “I would like to do something. It may not be typically Republican.”

In the federalism shell game that is the American system, the states would presumably ante up a good chunk of the $2 trillion while politicians in Washington would grab all the credit.

Of course, any D.C. discussion about giving America a 21st-century (or even late 20th-century) transportation system immediately prompts a bipartisan wave of fiscal responsibility. Funny that talk of $1 trillion deficits never seems to dominate the debate when Republicans are voting for unfunded tax cuts and when presidents of both parties are proposing new foreign military adventures.

But bring up new domestic spending (even when both Democrats and Republicans admit the pressing need) and suddenly both parties sound like Mr. Micawber from “David Copperfield”: “Annual income twenty pounds, annual expenditure twenty pounds ought and six, result misery.”

In truth, at a time of low interest rates, making 30-year investments in the American transportation system is a prudent reason for borrowing. Green-eyeshade calculations on Capitol Hill usually fail to take into account what the money would be spent for. As a result, Infrastructure Week is invariably followed by Can’t-Pay-For-It Week.

Returning to our alternate-universe history of the Trump years, the New York real estate hustler could have cemented his reputation as a master builder as soon as he moved from Trump Tower to the Oval Office. Instead of launching his self-defeating 2017 effort to repeal Obamacare and dutifully following the GOP party line on tax cuts, Trump could have scrambled political orthodoxy by moving toward the center with a massive transportation plan that rivaled Dwight Eisenhower’s Interstate Highway System.

Construction may be one of the few things that Trump actually thinks about — aside from money and his own underappreciated greatness.

In a February 2017 speech to the nation’s governors, Trump admitted that he worried whenever he was driven through the Lincoln and Holland tunnels under the Hudson River between New York and New Jersey. As Trump put it, “Every time I drive through, I say, ‘Man, I wonder how many people are hurt or injured when they’re driving at 40, 50 miles an hour through a tunnel and a tile falls off.’”

Sure, if Trump had, with the aid of Democrats, passed an infrastructure bill in early 2017, the president probably would have figured out a scheme for his sons to develop land at key highway off-ramps for luxury hotel-and-golf-course complexes. But at least after two years, the nation might have gotten something tangible from Trump beyond deranged tweets and a phantom border wall.

After Tuesday’s uncharacteristic outbreak of smiles and handshakes, some Democrats worried that any hint of congressional cooperation with Trump on infrastructure would fortify the president as he is reeling from the Mueller report and its aftershocks.

Senate Minority Leader Chuck Schumer tried to airbrush away these concerns when he said Tuesday, “We can come up with some good ideas on infrastructure … and the House and the Senate can proceed on its oversight responsibilities. The two are not mutually exclusive.”

A far better — and probably more politically adroit answer — would be to admit that Trump might briefly benefit from passing an infrastructure bill with decisive backing from the Democrats. But far more important than partisan political advantage is the notion that the biggest beneficiaries would be all Americans who have to drive, fly and take mass transit.

Trump’s political image is too set in stone (and no, it’s not the granite on Mount Rushmore) to be significantly altered by a legislative victory on infrastructure. The Democrats would also look selfish and petulant if they refused a $2 trillion deal that met most of their specifications.

Granted, cooperating with a president that you despise is not the way the game has been played on Capitol Hill in recent years. Mitch McConnell’s dismissal of Barack Obama (and ultimately Merrick Garland) is a model of scorched-earth partisanship.

Maybe Democrats should all get wristbands that read “WWMMD?” That acronym stands for “What Would Mitch McConnell Do?” And then, for the good of country, they should do the opposite, even if it means cooperating with Trump.

Walter Shapiro has covered the last 10 presidential campaigns. He is also a fellow at the Brennan Center for Justice at NYU and a lecturer in political science at Yale. Follow him on Twitter @MrWalterShapiro.