As confirmed by UK government spokesman, James Slack

  • If parliamentary legislation steps too far away from withdrawal agreement and political declaration, it does bring into question ratification

I think the second part is the more relevant issue to watch out for.

Opposition lawmakers are targeting amendments to Johnson's withdrawal agreement bill (assuming it goes through tomorrow) and if they succeed, it will be a major setback in trying to get a Brexit deal across the finish line.

The two main amendments that are being brought up right now are a customs union amendment and a second referendum amendment (these will be decided on Wednesday).

I am doubtful that either has a chance of passing but you just never know. Besides that, there is also the issue of timing as all the needed legislative measures need to be implemented over the next 10 days before the 31 October deadline.