The meaningful vote is probably the most unsafe of the Lords amendments to the EU (Withdrawal) Bill - because it tees up an unpredictable vote on the final terms of Brexit, towards the end of this year, and opens up the possibility that MPs could demand that ministers change policy, in the event the terms were rejected by the House, or no deal was reached in the talks with the EU.... they could even demand (drumroll) a second referendum...
Theresa May has suffered a ministerial resignation ahead of crunch Commons votes on Brexit, with Phillip Lee hitting out at the Government's "irresponsible" approach.
The House of Commons rejected the "meaningful vote" amendment to the withdrawal bill by a majority of 26, with 324 votes against the amendment, and 298 in favour of it.
In what is likely to be a highly-charged appearance before the backbench 1922 Committee on Monday, the Prime Minister is expected to say that the way MPs vote will send a message.
Conservative Remainer Anna Soubry and her colleague, Bernard Jenkin, a staunch Brexiteer, clashed on Twitter over what May had or had not promised regarding Grieve's compromise amendment.
A section of Labour MPs are expected to defy the official party position and vote in favor of a Lords amendment to keep the United Kingdom in a Norway-style trading arrangement, better known as the European Economic Area (EEA), post-Brexit. One says: "If we do not get what we were promised the Government will be defeated after we have amended the Bill in the Lords".
The government would not have sought a deal if it thought it had the votes to win, and they clearly blinked. "Any compromise [amendment] would have to be tabled by Govt in Lords".
Mr Buckland replied: "I entirely agree: the Government's policy is to achieve a deal because we are mindful of the points that he and others understand".
Mr Varadkar said while his government was making contingency plans for a hard Brexit, he insisted that preparatory work did not include for the provision of infrastructure on the 310-mile frontier with Northern Ireland.
That both sides have dug in so doggedly is a sign of how important - and divisive - the amendment is: it will determine whether parliament will have more power over the final stages of the Brexit deal, rather than the "take it or leave it vote" Mrs May has offered.
Labour and Conservative MPs are bitterly split over Prime Minister Theresa May's desire to keep Britain in the European Economic Area. Yes, it's a significant compromise but we live to fight another day. "We will wait and see the details of this concession and will hold ministers to account to ensure it lives up to the promises they have made to parliament".