Jon Tester (50.3%) got a much higher percentage of the vote than Hillary (36%) did. There's hardly anything mysterious about the vote numbers being much higher when a candidate gets 14% more of the vote.
The Democrat base turns out in force in any presidential election year. You see the REAL Democrat base, then.
My point is that Testes augmented the Democrat base mightily in an off-year "election".
Also, Testes managed to increase the TOTAL "turnout".
The 2016 presidential election in Montana had 484,986 total voters.
The 2018 US Senate "election" in Montana had 504,384 total "voters". Just THINK of the "enthusiasm"!!