There's really no evidence to support it being lean R. You could argue that the state's partisan lean makes it so, but that argument is flawed because downballot Dems routinely outrun the top of the ticket in Montana (see both of Bullock's victories, Tester's victory in 2012, etc.)
Montana is a Likely R/Safe R state at the Presidential level. This is a presidential year. Turnout will likely be high on both sides as Trump is polarizing, obviously, that is why Daines is likely to pull it out in my opinion. High turnout in a red state favors Rs in a state leans that way.
