As I've noted a number of times before, Stephen Bush of the New Statesman has an uncanny habit of making political predictions that either prove correct, or that prove a hell of a lot closer to being correct than the conventional wisdom of the moment. For example, although he wrongly predicted a Conservative majority at last year's general election, he nevertheless stuck his neck out and said that the Corbyn surge being picked up by the polls was real, at a time when most commentators were absolutely convinced it wasn't.
That said, I'm extremely unsure about the logic that has led him to conclude today that Theresa May can't use the threat of Corbyn as Prime Minister to bring Tory rebels into line, and that she will therefore probably be forced into making a U-turn on remaining in the EU customs union. Basically Bush makes the point that under the Fixed Term Parliaments Act, the Prime Minister can't designate a vote on the customs union as a vote of confidence in the government. So Remainer rebels would know that even if that vote was lost, and even if Theresa May felt compelled to resign as a result, the most likely outcome would be a Tory Brexiteer such as Michael Gove becoming PM, rather than Jeremy Corbyn. And then the new Brexiteer PM would have his hands tied by the pro-customs union arithmetic in the Commons anyway.
I think what this ignores is that staying in the customs union would cross enough of a red line for anti-European Tory MPs that they might actually prefer taking their chances with a snap general election, in the hope of getting a rebel-proof Tory majority that could overturn what had been decided. So if Gove or Boris Johnson stood in a leadership contest, they could find themselves under tremendous pressure to indicate that they will call an election in short order. And as we learned last year, if a Tory PM asks parliament to approve an early general election, the Labour opposition does not say no. What that means in the first instance is that pro-European Tories will know that rebelling on the customs union might lead to a general election that would carry not one but two possible risks - a) that Corbyn might win, or b) that the parliamentary arithmetic might become much more favourable for a Hard Brexit than is currently the case.