The average Maine bear is about the same size as the average Maine resident, about 140 - 150 lbs. Over bait at 30yards should present a very good opportunity for a revolver shot. A well placed, high quality bullet like the 180 grain partition from the 357 should do the job well. Although it's not the best bear medicine, it will work.
There are specific areas in Maine that have a larger average size bear. In these areas I would not consider the 357 enough. So depending on area, he should be ok.