I think you are confusing this with the referendum.

The nationalists want this deal to fail. This is the new trade deal with the EU that will govern once the UK leaves the EU. If it fails, the UK becomes a foreign government that becomes treated like any other foreign economy.

If you want to see a strong Europe, you want this to pass. If you want to see Europe splinter, you want it to fail. Vladimir Putin, for example, would love to see this fail. It will greatly weaken the UK's economy, possibly lead to the UK splitting up, and will toss a wrench in the European economic system since all the business/banking that goes through London could be put in jeopardy and will have to move to mainland Europe.