Granting the premise--climate change is the most crucial issue facing humanity at this time--who exactly ought to be raising this issue, who is guilty for fact it is not a significant election issue?

Not the candidates and their campaigns, for as Elagabalus says, we know that Trump is nuts on this issue as well as many others, so the only rational vote on this issue is Clinton. Given the absurd nature of this campaign, I wouldn't expect her to go there. 

The US Converence of Catholic Bishops, on the other hand, they should be blamed for not raising this issue as a factor. Because it is urgently relevant to an honest preoccupation with life. Because the universal Church has declared it the issue of our time. Because the bishops focus on abortion inclines those who care what the bishops think to vote for Trump and his party, who are on record as standing four square against trying to save our bacon on climate change.

May God have mercy on their anti-salvation stance.