European NATO allies are increasingly concerned that a major Republican defeat in Tuesday’s US midterm elections could prompt President Donald Trump to intensify his attacks on the alliance, potentially undermining European security at a time of growing tensions with Russia.

According to Politico, five diplomats, speaking on condition of anonymity because of the sensitivity of the discussions, warned that an electoral setback could push Trump towards more confrontational foreign policy decisions, including the withdrawal of thousands of additional American troops from Europe.

The central concern is that, if a Democratic-controlled Congress restricts his domestic agenda, Trump could increasingly turn his attention to foreign policy, where the US president, as commander-in-chief, enjoys considerably greater freedom of action.

Three diplomats suggested that such a shift could lead to substantial reductions in the American military presence across Europe.

“Typically, what happens with weakened presidents is a mixture of frustration and foreign policy adventurism,” a senior European defence official told Politico, adding that hostility towards NATO appeared to be a consistent feature of Trump’s worldview.

The White House rejected the suggestion that the president was undermining the alliance.

Deputy press secretary Olivia Wales insisted that Trump had “done more for NATO than anyone else”, pointing to agreements requiring allies to increase defence spending to 5% of GDP by 2035, the expansion of the US military presence in Greenland and his efforts to end Russia’s war in Ukraine.

However, she also stressed that Trump remained adamant that NATO allies should have supported the United States during its war with Iran.

“The president will never accept the managed decline of the West,” Wales said, adding that he would continue to pursue American interests both domestically and internationally.

The warnings come as Democrats are widely regarded as favourites to regain control of the House of Representatives, with a possible shift in the balance of power in the Senate also under consideration. Trump’s approval ratings, meanwhile, have fallen to record lows.

Relations between Washington and its European partners have already been strained by the US decision to cut off Starlink satellite services to Ukraine and Trump’s renewed rapprochement with Russian President Vladimir Putin in the weeks leading up to the election.

Any further deterioration could deal a serious blow to NATO as Russia steps up sabotage operations and other forms of hybrid warfare across Europe.

“He could become very angry,” one NATO diplomat warned. “And if that happens, he will be ruthless.”

American troops, NATO funding and Ukraine

One of the most immediate concerns involves the forthcoming review of American troop deployments in Europe, expected in December.

Three diplomats believe that, should Trump find his legislative priorities blocked by Congress, he could use the review to order a significantly larger withdrawal of US forces, presenting the move as a victory to supporters of his Make America Great Again movement.

Such a decision would potentially weaken NATO’s military capabilities on the continent and force European governments to shoulder a greater share of the alliance’s security responsibilities.

Oana Lungescu, a former senior NATO official and distinguished fellow at the Royal United Services Institute (RUSI), warned that the president could also use the continued deployment of American troops as leverage to pressure European governments into accepting a US-brokered peace agreement for Ukraine.

“There is a risk that all these things, the force posture review, Ukraine and the European security architecture, could be treated as part of a much broader deal with Putin,” she said, warning that such an approach could create an extremely dangerous situation for European security.

Diplomatic sources also suggested that Trump could reduce Washington’s contributions to NATO’s common budget, restrict the NATO-backed Prioritised Ukraine Requirements List (PURL) programme, which facilitates the supply of American weapons to Ukraine, and pursue closer relations with Moscow.

Together, these measures could significantly weaken Europe’s defence arrangements at a time when the continent remains heavily dependent on American military support.

Potential upheaval at the Pentagon

A severe Republican defeat could also trigger significant changes at the Pentagon, with consequences for US defence planning and military deployments overseas.

Defence Secretary Pete Hegseth has become increasingly unpopular in Congress, particularly among Democrats, because of his refusal to provide lawmakers with regular briefings on military budgets and operations.

His approach has fuelled speculation that he could be removed from his position following a major electoral setback.

A Democratic-controlled Congress would be expected to launch extensive investigations into the dismissal of dozens of senior generals and admirals, as well as Washington’s prolonged and increasingly problematic military involvement in Iran.

Such investigations could place further political pressure on Hegseth and the administration.

However, a change in Pentagon leadership could also complicate any attempt by Trump to rapidly withdraw troops from Europe.

If Hegseth were replaced, the transition to a new defence secretary could delay decisions on American force deployments for weeks or even months, potentially providing European allies with additional time to prepare.

Congressional restrictions and Europe’s military vulnerabilities

Although Trump has considerable authority over military matters, Congress retains important powers over defence spending and the deployment of American forces.

Under existing US legislation, the number of American military personnel stationed in Europe cannot fall below 76,000.

In theory, Democratic control of both chambers of Congress would therefore provide an additional safeguard against a drastic reduction in the US military presence.

However, experts question how effective those restrictions would be if the White House decided to challenge them.

Jennifer Kavanagh, a senior fellow at the Washington-based think tank Defense Priorities, said it was uncertain whether the statutory limits would survive legal scrutiny.

She also pointed to Trump’s willingness to disregard judicial rulings and congressional funding instructions.

According to Kavanagh, the administration has repeatedly ignored congressional directions on spending, directing resources towards activities that lawmakers have not authorised.

The consequences of a substantial American withdrawal would extend well beyond the number of troops stationed in Europe.

European NATO members would face serious difficulties replacing some of the military capabilities currently provided by the United States.

These include electronic warfare systems, air defence capabilities, intelligence gathering and reconnaissance, aerial refuelling and specialised logistical support.

Many of these capabilities are essential to NATO’s ability to respond rapidly to military threats and conduct large-scale operations.

Replacing them would require significant investment, industrial expansion and years of additional military planning.

Not all NATO allies expect a confrontation

Despite the mounting concerns, some European officials remain sceptical that Trump would take such drastic action against the alliance.

Montenegro’s Defence Minister Dragan Krapović told Politico that warnings about an American withdrawal from NATO had been circulating for years without materialising. “We keep hearing speculation about the US leaving NATO under Trump, which has not happened so far,” he said.

Krapović also emphasised the role of Congress in determining whether American troop numbers in Europe can be reduced.

Nevertheless, other diplomats remain deeply concerned about the unpredictability of the administration’s decisions following a possible Republican defeat.

A sixth NATO diplomat offered a particularly pessimistic assessment of the situation.

“To make reliable predictions, you need a subject of study who makes rational decisions,” the diplomat said. “I don’t think we are at that point with the current US president.”

With Americans heading to the polls on 3 November, European governments are therefore preparing not only for the political consequences of the election in Washington, but also for the possibility that its outcome could reshape the transatlantic security relationship and the future of NATO itself.