RNs shun labor deal....

How would this be done? I mean, are the nurses alone responsible for the cost? That would be a concern for me because it's expensive to keep up.

I don't know what they mean by "national certification".
My contract provides for extra pay for CCRN, not sure if this is the type of issue.

I do remember nurses at a hospital where friends work. The union and the RNs they had elected for the bargaining team recommended a "Yes" vote on a contract proposal.
The nurses voted "NO" and held a one day strike. It was about respect for senior nurses with decades of service to that hospital and about staffing on some units.
The union supported the nurses, they got what they wanted, and staffing was up to the ratio standards before the ratio law went into effect.