I think that:
1 - Polls in recent years have underestimated GOP voting
2 - There seems to be slight momentum in favor of the GOP, according to the latest polls
3 - Five Thirty Eight's model, for example, accounts for the fact that in Midterms, as we get closer to election day, there is a slight shift in favor of the party that's out of power, so in this case the GOP
4 - But all polls in Nevada are showing a statistical tie and this is a state that Biden carried by +2.4%, Hillary also carried by 2.4% and Jacky Rosen won the Senate race in 2018 by +5.0%. So it's been a consistently Democratic leaning state in recent years
Obviously I don't have a crystal ball but Nevada seems as close as it can get, could go either way by a small margin. In a way, the GOP needs the Nevada Senate win more than the Democrats do, as the GOP need to flip seats - the Dems don't.