Nice fiscal event you’ve got there, Prime Minister. Shame if something happened to it. The DUP is preparing to vote against the Budget should Theresa May ignore the party’s red lines over Brexit, Newsnight’s Nick Watt reports.

Although the spirit of our constitution is that losing a Budget means that the Prime Minister should resign, the letter of the Fixed Term Parliaments Act means that May would have no direct spur to – and crucially, means that the DUP can cause no end of havoc without bringing about an early election.

Are the DUP bluffing? Too much weight is given at Westminster to the DUP’s opposition to Labour as long as Jeremy Corbyn and John McDonnell are in charge. Yes, the two men’s historical associations with the IRA mean that the DUP will never support a government led by either of them.

But equally, the “U” in DUP doesn’t stand for “Don’t let Jeremy Corbyn become Prime Minister”; it stands for “Unionist”. And a deal that could unify the Conservative Party – Great Britain out of the customs union and single market while Northern Ireland remains aligned with the European Union – would put a stick of dynamite under the union between Great Britain and Northern Ireland. If after Brexit, any growing business or young entrepreneur in Northern Ireland finds it prohibitively different to expand to the rest of the United Kingdom but it remains simplicity itself to do business in Ireland, that’s the beginning of the end for the DUP’s political project.

Anyone who thinks that Arlene Foster and the DUP will put frustrating Jeremy Corbyn ahead of their constitutional aims is in for a nasty shock sooner or later.