Montana SB 279 has become law. It provides that in November 2016, the voters will vote on whether they want to amend the election law, to require a special election for U.S. Senate, when a seat becomes vacant. Current law lets the Governor appoint someone who holds the office until the next regularly-scheduled election.
The legislature could have simply passed an ordinary law making the same change, as was done this year in North Dakota. But apparently the Republican-majority legislature thought the Governor, who is a Democrat, might veto the bill. So they passed the idea as a referendum, which doesn’t need approval by the Governor. It seems overwhelmingly likely that the voters will approve the idea next year.