Donald Trump’s struggling golf resort in Scotland — and an airport nearby — are at the heart of a new dispute between the president and Congressional Democrats that some observers believe points to corruption in the Oval Office.

On Tuesday, the House Oversight Committee threatened to subpoena Pentagon documents that could shed light on repeated stays at Trump’s Turnberry resort by U.S. Air Force crews.

The most recent example was an Alaska-based Air National Guard crew that stopped to refuel at Glasgow’s Prestwick Airport, a half-hour drive from Turnberry, while on a routine cargo flight to Kuwait last March. The tab was paid by U.S. taxpayers.

In a letter sent Tuesday to Defense Secretary Mark Esper, Oversight chairman Rep. Elijah Cummings (D-MD) demanded immediate compliance with a request sent in June for “documents relating to taxpayer funds spent at President Trump’s [resort] … in apparent violation of the Domestic Emoluments Clause of the Constitution,” according to a committee press release, which notes that it has received no documents so far.

“If the Department does not begin producing the requested documents this week, the Committee will be forced to consider alternative measures to compel compliance, including a subpoena,” Cummings wrote.

A recent Politico report characterized the Oversight committee’s inquiry as “part of a broader, previously unreported probe into U.S. military expenditures at and around the Trump property in Scotland.”

“A senior Air Force official who was previously stationed at Elmendorf Air Force Base in Alaska — where the C-17 crew was based — said choosing to refuel in Glasgow and stay at a posh property a half hour away would be unusual for such a mission,” Politico said. “Typically, the official said, air crews stay on a military base while in transit or at nearby lodgings ‘unless all the hotels are booked or there is a Scottish sheep festival going on.’”

Trump denies everything.

In a tweet Monday, he wrote: “I know nothing about an Air Force plane landing at an airport (which I do not own and have nothing to do with) near Turnberry Resort (which I do own) in Scotland, and filling up with fuel, with the crew staying overnight at Turnberry (they have good taste!). NOTHING TO DO WITH ME.”

Others on Twitter weren’t convinced.

“Beginning in 2015, the U.S. Air Force contracted with Prestwick … to be one of the dozen military and civilian airfields in Europe where aircraft could refuel.,” says ABC News.

“Data provided by the Air Force shows that between 2015 and 2019 [U.S. military] aircraft stopped at Prestwick a total of 936 times, with 659 overnight stays. Those numbers have increased in recent years as more aircraft have made stops at the airport,” but Air Force officials say they don’t know how many times crews stayed at Turnberry.

In a letter in June, the House Oversight Committee told the Pentagon that since the fall of 2017, the military has spent $11 million on fuel at Prestwick — fuel that would have been somewhat cheaper at a U.S. base. That letter also cites a Guardian report that the airport provided cut-rate rooms and free rounds of golf at Turnberry for U.S. military members.

“Taken together,” Politico says, “the incidents raise the possibility that the military has helped keep Trump’s Turnberry resort afloat — the property lost $4.5 million in 2017, but revenue went up $3 million in 2018.”

And that increased revenue also increased suspicion among the House Democrats.

Despite Trump’s claim that none of this has anything to do with him, Cummings’ letter says, the president and his company “have had significant, direct, and detailed interactions with Prestwick Airport for years, including the President proclaiming that he was ‘thrilled to be partnering’ with the airport, the President personally trying to boost passenger traffic at the airport, and the Trump Organization negotiating room rates directly with the airport.”

The letter adds: “These and other actions by the President and his company reflect how critical this airport is to his financial bottom-line.”