For months, Prime Minister Theresa May has followed a simple plan for getting her Brexit deal passed by Parliament: keep bringing it back for a vote as the March 29 deadline draws closer and closer. Eventually, fear of the United Kingdom crashing out of the European Union without a deal, something widely viewed as a potential economic catastrophe, would stir enough MPs to back her plan.

Ms. May’s first two attempts went poorly, but she gave every indication of sticking to her plan. Then on Monday, Speaker of the House of Commons John Bercow announced that the deal had been dealt with and no new votes would be held on it unless it substantially changed. That derailed her strategy – and increased the likelihood of a no-deal Brexit. So Ms. May wrote to the EU asking for an extension to Britain’s departure date.

Today, European Council President Donald Tusk responded to Ms. May’s request. Yes, the EU would grant an extension – if Parliament voted for Ms. May’s deal. That doesn’t necessarily create a Catch-22. While Mr. Bercow did block revotes on Ms. May’s deal, the change in circumstance that Mr. Tusk’s ultimatum makes may reopen the door to a vote. But the ultimatum does increase the likelihood of parliamentary brinkmanship – and the risk of accidental no-deal.

Ms. May continues to promote her deal, hoping enough will support it to keep Britain from going over the Brexit cliff. Hard-liners in her party, meanwhile, race willingly toward the edge. Labour, though afraid of the chasm, thinks that the EU is waiting to catch Britain with an extension and may be willing to make a leap of faith. And Brussels has to decide whether to keep its nets out to save Britain.

With so many running at the cliff’s edge, the margin for error is vanishingly small.

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