Today’s vote is not a meaningful vote. But that doesn’t mean it is meaningless. If Theresa May’s motion is passed tonight, it will enable her to carry on saying to the EU that parliament supports her approach and that a legally binding change to the backstop is needed to get a deal through. But if May loses, then the EU will be confirmed in its view that May doesn’t have a stable majority for a deal even with changes to the backstop. Brussels will become surer and more vocal about its view that the way for the UK to pass the withdrawal agreement is for May to do a deal with Jeremy Corbyn on a customs union.
Right now, it isn’t certain what the result of tonight’s vote will be. The ERG is objecting to the motion on the grounds that they think it takes no deal off the table. Their argument is that it appears to accept the Spelman amendment which declared the House’s opposition to no deal. Whether the motion could be read as taking no deal off the table is, to my mind, debatable. But there’s no escaping the real-world consequences of a government defeat tonight. If you want changes to the backstop, then voting against this motion would be self-defeating.