Another year, another weird joint letter from Sen. Tom Cotton and his buddies to a foreign power.

In 2015, it was a terse warning to the mullahs in Tehran. The Iran nuclear deal was ‘nothing more than an executive agreement between President Obama and Ayatollah Khamenei,’ Cotton and 46 other Republican senators wrote. ‘The next president could revoke such an executive agreement with the stroke of a pen… We hope this letter enriches your knowledge of our constitutional system.’

In 2019, Cotton & co. turned not to foe but friend. ‘Congratulations again to you,’ Cotton and 44 others wrote Boris Johnson over the weekend. ‘Your great predecessor once averred that the British people had the heart of a lion, and he “had the luck to be called upon to give the roar” during their finest hour.’ In Cotton world, Iran is generally Hitler. But Britain – and Boris – get the Churchill treatment. The new prime minister, author of a recent Churchill biography, would no doubt be pleased.

Cotton’s aim is to reassure Boris about no deal Brexit, a real possibility as Britain careers towards its next big EU withdrawal deadline on October 31. Uncle Sam has your back, he wants to say. ‘If Britain leaves the EU with no deal,’ the signatories write, ‘we will work with our administration, your government, and our friends in the EU to minimize disruptions in critical matters such as international air travel, financial transactions, and the shipment of medicine, food, and other vital supplies.’ Draw your own conclusions on who those ‘friends in the EU’ might be.

‘I love it,’ Steve Bannon, former White House chief strategist and sometime consigliere to both Johnson and Cotton, told me.

Laugh all you want. But plays like this from Cotton and others are among the most effective tools in Congress’s box. The power of America’s legislature may be inscribed in the first article of the Constitution, but everybody knows that ‘the Hill’ is now the weakest branch of government. The power of the House and Senate’s 435 elected officials pales in comparison to that of the nine unelected jurists of the Supreme Court. It’s certainly no match for the Oval Office.

Policy letters may seem token gestures – and they often are – but they make policy formation more likely. These statements can hem a delegation in. On Iran in 2015, Cotton united hawks like Lindsey Graham and realists like Rand Paul, both presidential candidates that cycle. After the Republican party seizes power in 2016, the termination of the deal was far more likely.

So it will be with Cotton’s letter to Prime Minister Johnson. Donald Trump is the most protectionist president since Warren G. Harding. It’s no small matter that Cotton and crew just declared Britain effectively exempt from America First tariffs. Downing Street should take note.