(Duluth, MN)  --  Federal mediators have called negotiators for the nurses union and two Duluth hospitals back to the bargaining table, two days after nurses authorized a one-day strike on a date to be determined.  A tentative meeting between the union and Saint Mary's Duluth Clinic is set for next Wednesday and a similar session with St. Luke's Hospital is scheduled for Thursday.  The nurses union can issue a strike notice at any time, but leaders say their intent remains to avoid a strike and reach a settlement.