U.S. diesel averaged $6.50 a gallon on Friday, just shy of the record $6.53 set on Sept. 22 — and President Donald Trump says the White House is "very seriously" considering an export ban to bring prices down.

"We're thinking about it very seriously," Trump told a Fox News reporter on Sunday, while attending the Presidents Cup golf tournament in Illinois. "That can oftentimes lead to a little bit of an increase on gasoline for cars, so we're looking at it very seriously. We may do it."

The timing is political. Retail diesel at record highs is squeezing farmers, truck drivers and households weeks before the November midterm elections, and Trump said earlier this month a decision would come quickly "one way or another."

The policy detail remains fluid. Energy Secretary Chris Wright has said the administration is weighing restrictions rather than an outright ban, while Politico reported last week that the White House was preparing a plan to ban diesel exports for 90 days.

The commercial stakes are enormous, because the United States is the world's largest diesel exporter. It has also become the marginal supplier of diesel to Europe as flows from Russia and the Middle East have shrunk. Over the last couple of months, U.S. cargoes have covered roughly half of Europe's diesel imports, according to Benedict George, head of European product pricing at Argus Media.

Europe would take the hit first

Any U.S. restriction would likely send European diesel prices and premiums against crude "to a new unprecedented level," George told CNBC. European traders, he said, mostly doubt Washington will follow through, given how damaging the move would be for U.S. oil companies.

Morgan Stanley's commodity strategists reached a similar conclusion in a research note published Thursday. A U.S. export restriction would probably lower domestic diesel prices initially, they wrote — "but with potentially adverse reactions downstream."

"Not only would diesel prices be higher globally, but there could be a feedback loop to US gasoline prices as refinery runs adjust," the strategists said.

For shippers and carriers already absorbing elevated fuel surcharges, that dynamic matters directly. A ban would offer temporary relief at the U.S. pump while tightening product tanker demand and pushing European road-fuel costs higher, effectively exporting inflation to trading partners. The American Petroleum Institute moved within days to contest the idea.

"Restricting U.S. energy exports would only compound the problem — exacerbating refining challenges and ultimately hurting consumers," API CEO Mike Sommers said in a statement. "The answer is more supply and more flexibility — not new restrictions that risk making a difficult situation worse."

War-driven crunch

The price surge has two engines: hostilities between the U.S. and Iran, and between Russia and Ukraine, both disrupting vital oil and fuel trade routes. Ukrainian drone strikes on Russian refineries — which Kyiv considers legitimate military targets — have knocked out processing capacity and forced buyers worldwide to compete for replacement barrels. Trump has urged Ukrainian President Volodymyr Zelenskyy to stop targeting the refineries, saying the attacks are "hurting the world."

Those strikes have transformed diesel from one of several big problems into the core one, according to Argus' George. "It's the biggest problem for the global oil system, whereas before it was one of several very big problems," he said.

Ukraine, bracing for another Russian assault on its energy grid and what it expects to be an extremely difficult winter, shows no sign of abandoning the campaign.

No forecast horizon

Uncertainty has reached the point where traders have stopped building outlooks altogether. "One trader was saying to me recently that he doesn't bother trying to forecast now because it feels like a waste of effort. You have literally no idea what is going to happen," George said.

On the U.S. side, he noted, any restriction would likely be short-lived. "I think all anybody has talked about is a short-term measure, so two or three months at an absolute most … so there is a kind of time horizon on the U.S. restriction of exports, if it were to happen," he said. "But on the Russia-Ukraine conflict, who knows? I mean, literally nobody knows. Nothing has worked so far to resolve that situation and similarly for the semi-closure of the Strait of Hormuz."

For now, nothing is decided. George cautioned that "there is no measure yet and it's very unclear whether there will be a measure at all." If Trump does act, expect a 90-day shock to European supply and a counter-reaction in U.S. gasoline — and if he doesn't, the Russia-Ukraine war and the Strait of Hormuz remain the swing factors keeping diesel near record levels.