Addressing the House of Commons on Monday, British Prime Minister Theresa May delayed a vote on her Brexit withdrawal or transition agreement with the European Union. The vote was originally planned for Tuesday but will now take place at an as yet undetermined date before Jan. 21, 2019.

May's delay of this vote is a big deal. It proves that her plan lacks the support of the House of Commons. Still, it was necessary for a simple reason: Had the Commons voted on Tuesday, May's plan would have been defeated and her position as prime minister imperiled to the point of a leadership challenge.

The operative question now is how May intends to attract enough votes to win a vote in the future?

The answer is far from clear. May says she'll focus on getting improved clarity over what any border arrangement might look like if Britain and the EU have been unable to reach a post-Brexit transition arrangement when the transition period concludes on Dec. 31, 2020. Known as the "backstop," and applying only if Britain and the EU had not agreed a trade deal by the end of the transition period, it would involve some form of border control at the United Kingdom's Northern Ireland border with the EU member state, the Republic of Ireland. But it's unclear whether gaining such clarity would give May the votes she needs.

And it gets worse for May. Because while British parliamentarians want to avoid any formal border with the EU, the EU demands a backstop arrangement that would allow EU customs inspectors to check goods passing across the border. The challenge for May is that her government relies on the support — and would probably collapse absent it — of a Northern Irish party, the DUP, that opposes any kind of formal border arrangement.

May's ultimate challenge, however, is how she gets any more concessions out of the EU that might allow her to win parliamentary support for her Brexit deal. And absent that parliamentary support, a Hard Brexit, or Brexit absent any transition agreement, would follow. The vast majority of economists believe such an outcome would risk sending Britain spiraling into a recession.