Okay I shouldn't have said Safe D but the chances Heller wins are very low, like 10-15 %.
Don't underestimate him. He got elected in Nevada in 2012 despite the fact that Barack Obama won that state by 7 percentage points over Mitt Romney that same year. So he is able to win tough senate races in Nevada even if it is the only 2018 senate battleground state won by Hillary Clinton in 2016.
He won against a very flawed democratic candidate who was under a criminal investigation and even with that, there was an option that said none of the above, which appears to be what democrats/left leaning independents selected who couldn't morally vote for a corrupt person like Berkley but they couldn't bring themselves to vote for Heller either. Had that option not been there, those dems would have reluctantly backed Berkley and he would have lost. And he got around the same percentage as Mitt Romney, the democrat just did worse then Obama. The whole narrative that he's some great candidate is overblown and misleading
No one said that he is some great Candidate BUT he is an Incumbent Senator and that matters.
Incumbency doesnt make a senator invincible. It can help, but only somewhat, and the effect of incumbency is highly based off of your personal popularity. This is why Baker is invincible, yet many D senators in 2010 and 2014 were not.