Can Spain stop the draft Brexit agreement from happening?
Technically, no. However, from a unspoken rule perspective, they very well may be able to.
This Sunday's vote for the withdrawal agreement only requires a qualified majority among EU27 members to succeed and that is almost a given now. From a technical point of view, Spain would just represent one vote and the needed votes to back the agreement is 20.
However, the issue here is that in general EU27 members are unwilling to sign off on anything unless there is unanimity in the decision. Even more so when one particular nation (in this case Spain) has a major source of grievance on the agreement.
The issue with not coming to a common ground here is that eventually when put to a vote, Spain can choose to vote down an eventual trade deal between the EU and UK. And that vote counts because such a scenario would require a unanimous decision.
Hence, if the Gibraltar issue cannot be resolved by Sunday, essentially everything else will be at risk as Spain can just vote down the future trade deal when the time comes.
