UK Prime Minister Boris Johnson. Picture: POOL VIA RETUERS/DANIEL LEAL-OLIVAS
UK Prime Minister Boris Johnson. Picture: POOL VIA RETUERS/DANIEL LEAL-OLIVAS

If, as reports suggest, British and EU negotiators are inching toward a point where they can present a Brexit deal for approval, the UK parliament will soon have a historic decision to make on whether to back it. For many MPs, it would be a choice between two different types of misery.

Opponents of Brexit, or this particular deal, would voice strong objections to whatever Johnson brings back (if indeed he pushes ahead with an agreement laden with British concessions). But would Brexit-weary voters forgive a party that turned down a deal now? Johnson is betting they wouldn’t.

Winning support from his own Conservative MPs should be more straightforward for Johnson than it was for Theresa May, his predecessor in No 10. Lee Rowley, a Tory MP who opposed May’s deal three times, was speaking for others too when he said on Monday: “For the health of our democracy and to restore faith in this most venerable of institutions, in my view we simply must get Brexit done.”

The so-called Spartans among Conservative MPs who refused to support May’s withdrawal deal have given signs they’ll back their new leader. They’re deeply worried that further delays to Brexit will hurt the Conservatives’ chances at an imminent general election and increase the likelihood of the UK not quitting the EU at all.

It helps that some arch-Brexiters are now on the government payroll. Jacob Rees-Mogg, leader of the House of Commons, has even suggested that if Johnson comes back with terms like those he described previously as “cretinous” when they had May’s name on them, he would probably support them.

That wall of endorsement might crumble if Northern Ireland’s Democratic Unionist Party — the Conservatives’ parliamentary allies — withhold their blessing, and EU officials believe Johnson won’t give the green light to a deal unless he’s sure the DUP will back it. Arlene Foster, the party’s leader, said of Johnson's potential deal on Tuesday evening: “It would be fair to indicate gaps remain and further work is required.”

The prime minister’s proposed concessions to Brussels are thought to include customs checks between the UK mainland and Northern Ireland, anathema to unionists.

But maybe the DUP can be bought off by promises of billions of extra cash from Westminster for Northern Ireland. And Johnson will have been buoyed somewhat by potential support for his putative deal from some Brexiter MPs in the opposition Labour Party.

Indeed, Labour’s mightily complicated Brexit position will look even more awkward if Johnson came back to Parliament with a deal and DUP support. Party leader Jeremy Corbyn’s Brexit strategy has assumed an almost infinite level of public patience: First win a general election (which will have to take place soon given Johnson’s lack of a majority), then renegotiate the Brexit deal and then hold a referendum on whether to remain or leave with Labour’s deal.

With the British public exhausted by Brexit, a Johnson deal would look like more of an election winner for the Tories.

Any deal vote in Parliament will almost certainly include an amendment demanding the agreement be put to the people in a confirmatory referendum. Some Labour MPs favour the party backing such a referendum before agreeing to a new general election. But that would require another long extension of the Brexit process, again not something that’s going to be loved by many British voters.

The position of the centrist Liberal Democrats, revived under leader Jo Swinson as the Stop Brexit party, will be important. The Lib Dems support a “people’s vote” on any agreed Brexit deal, although their preference is to revoke Brexit altogether. But if getting a referendum required a vote of no-confidence in Johnson, Swinson has refused to back even a temporary replacement government with Corbyn in charge. So the Labour leader would have to let someone else become prime minister or Swinson would have to accept him in Downing Street. Neither outcome looks likely.

In other words, all parties are considering not just the terms of Johnson’s possible deal but the terms on which they will fight the election. A vote for a deal would be a vote to end this phase of the Brexit negotiations and pave the way for the general election, although it would strengthen Johnson going into it. A vote against a deal would probably mean an extension from the EU to the October 31 Brexit deadline and more uncertainty, perhaps still including an election. Voters again may reward Johnson for his efforts to break the impasse and penalise MPs who got in the way.

The stakes are high for another reason too: an exit deal is just the beginning. A bigger, more important, negotiation on the future UK-EU trading relationship starts after that. The next government will shape that as well as Britain’s economic direction generally.

For the past three years, legislators have agreed repeatedly that they don’t want a no-deal exit, but they haven’t found a majority for anything else. Even with the relief that might come from a deal, it’s not hard to see why they might struggle to say yes now.

• Raphael writes editorials on European politics and economics for Bloomberg Opinion.

Bloomberg