The United Kingdom voted in 2016 to leave the European Union effective March 31, 2019. Parliament, pundits, and the public cannot agree on how Brexit, as it is known, should be achieved.

As the battle rages, President Trump should provide our ally with a clarity and comfort by promising that whatever exit Britain makes from the EU, it can have a rapid post-Brexit trade deal with the U.S.

Trump believes in "America First," so he would be the first with a deal in the U.S. national interest. Of America's top 10 export destinations, Britain is the only one that imports more goods from us — $55 billion worth a year — than it exports to us. This surplus reveals Britain's great demand for U.S. creations. It also makes obvious sense to double down on this market. A rapid trade deal would foster a grand new opening for American companies to sell more of their products. It would create jobs in America and happy consumers in Britain. It would also help reduce the current account deficit, which Trump so often laments.

Trump has signaled opposition to a trade deal if Britain also agrees to a regulatory deal with the EU. But while these concerns are legitimate, London says it won't allow any EU framework to obstruct American interests under any U.K.-U.S. deal.

Britain also needs our help. With Prime Minister Theresa May facing resistance to her Brexit plan and no clear path toward an alternative plan yet apparent, we should reassure our closest friend that Washington has its economic back. This will help if what happens is "hard Brexit" — Brexit without a deal with Brussels — which is what will happen under current law at 11 p.m. on March 29 if no sides agree to changes.

Some analysts fear that hard Brexit would prompt capital flight from Britain, bringing recession. This would undermine American exporters and British stability. That stability also matters to America. If May falls from power, the chances of Labour's Jeremy Corbyn, an anti-American and admirer of Leon Trotsky, becoming prime minister are high. Corbyn in 10 Downing Street is a recipe for Britain's diplomatic, intelligence, and military abandonment of American interests.

There's more at stake than trade opportunity and political stability. There's the idea of what trading relationships should look like in the 21st century. The EU offers a very different kind of trading model than what a good U.S.-U.K. deal would look like.

Where the U.S.-U.K. deal would focus on maximum economic competition and market access, EU trading favors protection, sporadic and limited access, and national subsidies. That central planning limits effective capital flows, limits growth, and drives up prices. A U.S.-U.K. trade deal could serve as an alternative model. It would show how competition fosters the best products entering the market most efficiently, and how increased product diversity means satisfied consumers. In short, it would be a model of free enterprise in contrast to the continent's central planning.

Brexit's future is in doubt. America has a role to play. A trade deal with Britain makes sense. Trump should act now.