For once, Boris Johnson might be onto something.

Britain’s bombastic former foreign secretary warned over the weekend that the country was “on the verge of making a historic mistake.” Johnson, an ardent supporter of a hardline approach to leaving the European Union, meant that Prime Minister Theresa May gave away too much in the exit deal she closed Sunday with the EU.

Many people agree with Johnson that leaving on the terms May negotiated would be a blunder. Far fewer agree with him on exactly why it would be a blunder. Polling has been tricky, but most recent surveys indicate that if they were given a do-over, Brits probably wouldn’t vote to leave at all – a small majority in favor of leaving in 2016 appears to have flipped to a slight majority in favor of staying. Others want to leave but don’t like this specific deal. The best anyone can say of it is that it’s the best May could get. The EU insists it won’t renegotiate.

Making matters worse, the path to any option other than this deal – or crashing out of the EU altogether at precisely 11 p.m. local time on March 29 – is getting narrower and rockier by the day.  

Sunday’s agreement governs what would happen during a 21-month transition period while Britain and the EU negotiate details of their new relationship. Initially, little would change. Britain would continue paying into the EU budget, for instance. Johnson and other Brexiteers favor a faster, sharper break with the EU.

While polling is close on the question of whether Britain should stay or go, it has been much clearer on a few other questions. Overall, respondents this year have said May’s government has done a poor job of negotiating and wasn’t going to get a good deal. A plurality thinks Brexit will be bad for the economy.

May appears to have dodged one big problem this past week. The hardline Brexit faction of her Conservative Party failed to come up with enough support to demand a vote of confidence in Parliament.

But pressure is building for a new referendum, this time including the specifics of the deal. May adamantly rejects that. Labor leader Jeremy Corbyn is waffling about it, but support seems to be growing within his party. The Liberal Democrats and Scottish National Party want to vote again.

So what happens now?

It’s complicated. Sometime in the next several weeks — the date Dec. 12 has been mentioned — May must bring the agreement to Parliament for approval. Most experts think she will fail. May only commands a majority in the 650-seat House of Commons with the help of 10 MPs from Northern Ireland’s Democratic Unionist Party, which is against the agreement. Among May’s own Conservatives, BBC says it has counted 81 who object to the plan. Add in members of opposition parties who want to remain in the EU, or just want to bring down the government, and May is probably well short of what she needs.

The prime minister will campaign hard, and the leadership can twist some arms. But the betting is that she still comes up short. Meanwhile, the clock will be ticking.

BBC political correspondent Ben Wright says May would then have three weeks to come back to Parliament with a plan for how to proceed, plus another week to propose specific action – at which point Parliament could weigh in again. By now, we’d be well into January. Politicians and business leaders would be starting to panic, and May might be tempted to bring back essentially the same proposal and dare Parliament to vote it down again. In any case, Wright says, MPs probably wouldn’t be allowed to amend the government’s plan, and defeating it wouldn’t stop Brexit.

On the other hand, the reality of a hard exit might create a stampede toward a second referendum. But that can’t happen unless the government proposes it (which May won’t do) and Parliament approves. And there probably would be no time to organize a general election, Corbyn’s preferred solution, before the end of March.

That’s a sobering scenario for one of the most consequential decisions Britain will make in many years. Wright quotes Justine Greening, a former Cabinet member from May’s party who now favors a second referendum, as saying the split is so bad that there is “no majority for any route forward.”

If the prime minister can work some political magic and get her plan through, Britain would survive leaving the EU under the terms she negotiated. It wouldn’t be pretty, and it wouldn’t be necessary, but it would be doable.

If politicians suddenly started coalescing around a second referendum, that could work, too. The Wall Street Journal’s Joseph C. Sternberg warns of establishing a precedent in which any contentious issue goes to a nationwide vote – and in which the losers automatically start agitating for a revote. That’s not a good way to do business, but still preferable to the alternatives.

Instead, Britain risks ending up with the worst possible outcome, crashing out of the European Union only because it can’t agree on what else to do. Time is running out.