Early on Wednesday morning the results will start to trickle in from the most consequential US presidential election in a generation.

The result will set the path for the US’s relationship with a UK led by its first Labour government in 14 years. Keir Starmer and his team have sought to prepare themselves for either eventuality: a Kamala Harris victory, with its easier, more predictable consequences, or a second Donald Trump administration and the volatility that would bring.

Here’s a look at what a Harris or Trump win would mean for UK-US relations.

Politics

A Trump victory would be politically awkward for the Labour government, which has close links to the Democrats. The Trump campaign filed a legal complaint last month accusing the Labour party of interfering in the US election after it emerged that 100 current and former staffers were heading to the US to campaign for Harris.

Starmer has sought to separate party and government, and has made overtures to Trump since becoming prime minister in July. He was one of a few world leaders to speak directly to the former president after he survived an assassination attempt, and they met in New York in September for a two-hour dinner.

Despite his efforts Starmer has not succeeded in meeting Harris before the election, but they are far closer politically. Morgan McSweeney, Starmer’s most senior adviser, and Deborah Mattinson, one of his election strategists, are among several top Labour aides who attended the Democratic National Convention in Chicago in August.

Security and defence

Nowhere is there a starker contrast between the two presidential candidates. By and large, a Harris win offers continuity on Nato and Ukraine. She has been strongly critical of Vladimir Putin, and her running mate, Tim Walz, the governor of Minnesota, is seen as a staunch Ukraine ally. Soon after Russia launched its full-scale invasion, Walz signed an order requiring state agencies to axe contracts with Russia and banned them from agreeing new ones.

A Trump victory would have huge consequences for the future of Nato, which is spearheading Ukraine’s defence. In the past Trump has threatened to leave the alliance, in a move designed to put pressure on US allies to increase their defence spending. He has criticised the amount the US spends on supporting Ukraine’s defence and censured its president, Volodymyr Zelenskyy, for not making concessions to Russia. He has made clear he wants to see a quick resolution to the war and claimed he could end it in a day – but a ceasefire that benefits Vladimir Putin poses a major risk to European security.

Climate and environment

The US has repeatedly, under Trump and Biden, insisted agriculture must be core to any post-Brexit trade agreement, but the British public has been promised that chlorinated meat and hormone-fed beef will be excluded – an impasse that will stymy Starmer, but opens up opportunities for Kemi Badenoch should she decide to take up the cudgels for free trade over food safety.

Starmer used his first speech to fellow world leaders at the UN general assembly in September to present his government as a global leader on the climate, and will repeat that next week at the Cop29 summit. Harris would be a powerful ally; a Trump win would send the prospects of limiting global heating to safe limits into a tailspin.

Trade

Nearly five years after Brexit, the prospect of that US-UK trade deal looks more distant than ever. In the summer of 2023, Rishi Sunak conceded that agreeing one was not a priority for either country and had not been for a while.

The fact that the Democrats and Republicans remain notably protectionist in their economic policy does not bode well for hopes of a free trade agreement under Labour. Both US parties are keen to appeal to voters in battleground states in the rust belt who have suffered from deindustrialisation.

Trump’s economic agenda in particular is built around tariffs – he has suggested that under his presidency, all goods imported into the US could be slapped with a 10% tariff and that vehicles could be hit with a 100% tariff, affecting UK exporters. While Harris isn’t nearly as protectionist in her approach, a bilateral trade agreement with the UK is unlikely to be a priority for her.