Boris Johnson is the last person Tory MPs should listen to on Northern Ireland

Conservative MPs need to get a grip. If they want to lose the election, they are on course and doing a fine job. But it is their democratic duty to try to present the electorate with a competitive choice, so they ought to give Rishi Sunak the support he deserves.

The idea of voting down an attempt to fix the problems with the Northern Ireland protocol ought to be dismissed as fundamentally unserious. In particular, the idea that Boris Johnson should be the arbiter of what works is ridiculous – it was he who negotiated the protocol, and who then decided, the moment we left the EU, that it was a terrible deal. A period of silence on his part would be welcome.

It was Johnson, his fellow Tory MPs should remember, who didn’t give Ukraine fighter jets when he was prime minister, and who now criticises Sunak for not giving them jets. It was he who had enough support to contest a vote of the party membership in order to return as prime minister, but decided that “you can’t govern effectively unless you have a united party in parliament”. He was right. He couldn’t. Why, then, should anyone give him the time of day when he tries to make sure that Sunak can’t govern effectively either?

When Sunak tries to put right the mistakes he inherited from Johnson, he should be supported. Some Tory MPs might have been bamboozled by Johnson before the last election, believing that he had secured a good deal for Northern Ireland. But now that David Frost, Johnson’s chief negotiator, has admitted in effect that it was a bad deal agreed under duress in order to force an election and get Brexit done, they should know better. Especially as the Democratic Unionist Party (DUP) always said it was a bad deal.

Sunak and his team are now engaged in a serious attempt to produce a workable compromise that is acceptable to the DUP – something that Johnson abandoned because it was too difficult, cutting and running and hoping he could sort out the details later.

What is left of the European Research Group (ERG) on the Conservative back benches claims to detect an attempt to sell out the true principles of Brexit. This is unconvincing. There may be those who think that the only solution to the Irish border problem would be for the UK to rejoin the EU single market, but Sunak is not one of them. He was a Brexiteer before Johnson wrote his articles making opposing arguments to decide what he thought. He kept Chris Heaton-Harris and Steve Baker – both Brexiteers – in post as ministers in the Northern Ireland Office.

Sunak knows, too, that the only way to fix the protocol, in a way that would restore devolved government in Northern Ireland, is, by definition, to secure the agreement of the DUP, the largest of the unionist parties. There have been persistent suggestions that the government might press ahead without the DUP and the rump of ERG Tory MPs, but these make no sense.

If the DUP doesn’t agree to new trade rules, then the Northern Ireland Assembly will remain in suspension. The UK and the EU can tinker with the customs rules, and even agree on red and green lanes, but the political deadlock will remain.

Which is why I assumed that the prime minister had already done the deal with the DUP, and that the rest of the negotiations would be a show designed to sell it to widening concentric circles of what are unfortunately known as stakeholders. Sir Jeffrey Donaldson, the DUP leader, certainly seemed positive after his meeting with Sunak on Friday, saying “progress has been made” although “further work is required”.

I thought this was code for “deal done, subject to the pretence of last-moment concessions”. The prime minister seemed to think that he had received an indication from Donaldson, because he allowed it to be known that he intended to unveil the deal today, and even, possibly, put legislation to the Commons. And yet the entire history of Northern Ireland negotiations, and specifically of talks with the DUP, should have said that this was liable to go wrong.

Without even a text to go on, it was dangerous to take anything for granted. Thus Sammy Wilson, the DUP chief whip, who on Friday said the party was in “problem-solving mode”, yesterday said progress was “slim” and accused ministers of going into talks with the EU in “an attitude of defeat”.

What makes the supposed deal even harder to assess is that nothing has leaked about how the fundamental problems might be solved. Even if the DUP’s objections to the protocol could be met, the party seems unlikely to go back into government under Michelle O’Neill of Sinn Fein as first minister. But with Sinn Fein the largest party after last year’s assembly elections, that is what is supposed to happen, according to the St Andrews Agreement – which updated the Good Friday/Belfast Agreement in 2006.

If Sunak can solve yet another apparently insoluble Northern Ireland problem, he deserves the wholehearted support of his own party rather than destabilising interference from his predecessor – a predecessor who signed a deal that he admitted, before the ink was dry, needed to be redrafted from scratch.