“He’s not going to follow rules that he thinks are stupid rules,” said a former senior White House official in Trump’s first term.

WASHINGTON — Foreign leaders have lined up to speak with him. He has rattled Mexico and Canada with threats of steep tariffs and warned there would be “hell to pay” for militants in Gaza unless they release the hostages by the time he’s sworn in.

That won’t happen for another 45 days, but Donald Trump, the president-in-waiting, isn’t shying away from acting like the president-in-reality.

Trump can’t sign a bill or issue an executive order yet, but he is crowding out Joe Biden as the sitting president winds down his term and steadily recedes from public view. In two foreign trips since the election, Biden has answered all of two questions from reporters.

He has been left to kibitz about Trump’s pronouncements — “I hope he rethinks it,” he said of Trump’s plan to impose 25% tariffs on Canada and Mexico — rather than drive an agenda of his own.

As for Trump, “his view is that he’s not going to follow rules that he thinks are stupid rules,” said a former senior White House official in Trump’s first term. “His view is these are hostages and if he can help bring them home, then why would he follow protocol if it’s going to impact peoples’ lives?”

At this point, Trump is “already basically running things, and he’s not even president yet,” the person added.

Trump’s penchant for plunging into current affairs is testing the one-at-a-time dictum that presidents are supposed to honor but, for reasons of political expediency or practical necessity, usually don’t.

“He would probably argue, ‘I’m more than a private citizen at this point; I’m president-elect, and I’m going to have all the powers of the presidency in a couple of months.’ But that doesn’t mean you get to start before you’re inaugurated,” said Rep. Jared Huffman, D-Calif.

This weekend, Trump will join French President Emmanuel Macron in Paris for the reopening of Notre Dame Cathedral five years after it was devastated by fire. Biden was invited but opted not to attend, a White House official said.

Trump’s return to the world stage after a four-year hiatus, coupled with news about his hires, has overshadowed Biden’s trip to sub-Saharan Africa this week, in which he faced fallout from the sweeping pardon he gave his son Hunter.

“Given the weakness of the current president and the speed at which things develop in the modern world, Trump is, in effect, a presumptive president,” said Newt Gingrich, a Republican former House speaker and a Trump ally. “Certainly, foreign governments are treating him that way.”

If Biden is unhappy about being upstaged, he hasn’t offered any hints. In fact, Trump’s interventions may prove helpful to the degree they complement Biden’s larger goals.

A senior Biden administration official cited Trump’s social media post warning of “hell to pay” unless the hostages are freed from Gaza by Inauguration Day. That message doesn’t hurt the Biden administration’s efforts to secure the hostages’ release in exchange for a ceasefire, the official said.

A president-elect’s first and overriding task is to build a Cabinet and a senior White House team ahead of the four-year term. But Trump and a few of his predecessors haven’t hesitated to dip a toe into real-time crises if so moved.

As he prepared to take office in 2016, Trump tried to salvage jobs that the corporate parent of Carrier, a heating and air conditioning company, had planned to move from Indiana to Mexico.

And he weighed in on Boeing’s plans to build the next generation of presidential aircraft, decrying the expense. “Cancel order!” he tweeted as president-elect in 2016.

Internet sleuths believe they have found the jacket worn by the gunman who killed UnitedHealthcare CEO Brian Thompson … and now it is morbidly flying off the shelves.

On Reddit, a user speculated the suspect’s jacket was a Sherpa Lined Two Picket Hooded Trucker Jacket by Levi’s … sold at Macy’s for the retail price of $225.

The jacket’s popularity has since spread like wildfire on the company’s website … where more than 6,000 people were viewing the jacket at the same time — and nearly 700 were sold in the past 48 hours, according to an item popularity tool on Macy’s site.

As you know, Thompson was fatally shot in Manhattan on Wednesday … and the nationwide manhunt for his killer continues.

NYPD Chief of Detectives Joseph Kenny told reporters Friday that detectives are looking into the possibility the gunman used a veterinary gun … a larger gun commonly used on ranches and farms — but the gun hasn’t been recovered.

However, a backpack suspected of belonging to the shooter was found in Central Park. As TMZ reported … the backpack appears to be an Everyday Backpack from Peak Designs … a bag meant for serious photographers but designed for casual use, also.

We have reached out to Macy’s regarding the jacket … so far, no word back.

• Credits: TMZ