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.