It is material and formal collaboration in mass murder.
I disagree that deciding not to vote for either presidential candidate us a mortal sin.
In itself, it is not even remote material cooperation in evil, nor is it, by any stretch of the imagination, formal cooperation.
If abstaining from the top line is *motivated*, in whole or in part, by approval of gravely intrinsically wrong things (i.e. abortion,sodomy, sacrilege, attack on the Church), then we’re in the mortal sin category through intending grave evil.
But if it’s motivated by the desire to avoid complicity in grave evil, the intent is just, and the choice is an example of prudential judgement.
“Not voting” is not the same as cooperation, just as taking a vow of silence is not the same as lying.
Is it your contention that voting is always morally obligatory?