Donald Trump has waded into British politics, telling LBC host and Brexit Party CEO Nigel Farage that Boris Johnson's Brexit withdrawal agreement means that a US-UK trade deal can't be done, and adding that Johnson is “the exact right guy for the times”, while Jeremy Corbyn would be “so bad” as prime minister.

It's a double headache for Johnson: it gives Farage and the Brexit Party an opportunity to argue that Johnson's deal is a rotten one that is Brexit in name only, and it gives Labour and Corbyn the opportunity to pivot away from Brexit, a question on which the country is split almost exactly in half, and towards Trump, one which unites more than two-thirds of British voters in varying degrees of contempt

The reality is that Trump is wrong: Johnson's deal leaves plenty of room for a deep US-UK trade deal. It's Johnson's promises as far as food and agriculture go that will have to be abandoned if he wants a meaningful trade deal with the United States.

But it leaves Downing Street in an awkward position: of at once trying to defend their deal and at the same time trying to minimise the ability of Labour to equate Johnson and Trump. Their big consolation is that because their opposition parties have different incentives, the negative messages are drowned out. Corbyn, Jo Swinson and Nicola Sturgeon want voters to think that Johnson's deal is one that opens the UK up to becoming a wholly-owned subsidiary of Trump, Inc, while Farage wants voters to think that there is no prospect of a trade deal with anyone under the terms of Johnson's withdrawal agreement. The electorate might simply conclude that if Johnson is being attacked from both sides, he must be doing something right: or voters might unite in thinking Johnson's deal is a rotten one despite doing so for verty different reasons.

It speaks to the big known unknown of the general election: we don't know what it's about yet. Is it about the question of Brexit, which carries a heavy risk but a big prize for Johnson and Swinson? Is it about the NHS or the condition of the public realm, a contest which Corbyn believes he can win? Is it about finding the most reliable way to prevent a Trump-aligned Conservative Party from winning? Is it about finding a lifeboat for Scotland or preventing a second independence referendum? Is it about Jennifer Arcuri and the cost of IT lessons in London these days? Or is it about some as yet unknown event that could change the contest entirely?