Nurses at Allina Health's Twin Cities facilities are in their final day of scheduled contract talks with the health system.
Their contract expires at midnight Tuesday.
Allina says it is committed to reaching a "fair and sustainable" contract agreement with its nurses. The Minnesota Nurses Association declined to comment on the talks.
In written communications, both sides acknowledge much of the dispute is over health benefits. Allina wants to shift 4800 nurses at Abbot Northwestern, Phillips Eye Institute and Mercy, Unity and United Hospitals into its corporate health plan.
The nurses say switching plans would impose millions of dollars of additional costs on union members.
In a negotiations update posted Sunday, the Minnesota Nurses Association accuses Allina of not giving "a valid reason" for shifting health care costs to union members.
Allina says the proposed change would move nurses, "to the same competitive and affordable health plans," offered to all of its other employees.
If the rank and file authorize a strike, the union would have to give Allina ten days' notice before a walk out.