Whenever there is a candidate who, like Hillary, promises to do monstrous evils, such as genocide or mass abortion, it is a strict, grave obligation to vote for a candidate who will not commit crimes of equal gravity.
In this election, the only candidate who can possibly defeat Hillary is Trump. Thus, voting for Trump is a strict, grave obligation.
If Trump IS going to do some evil, putting him in office is NOT formal cooperation in that evil, because the purpose of voting for Trump is to keep Hillary out.
As it happens, nothing that Trump is committed to doing is evil.
Assuming Trump is a serial groper—for which there is no evidence—this is relevant to voting ONLY if it is demonstrable that all serial gropers, once in office, gravely damage the common good more than lifelong abortion fanatics who get into office.
(BTW: By far the MOST idiotic statement by any bishop this cycle is Conley’s statement that Catholics should not believe themselves obligated to vote for anyone in order to keep someone out of office. That is absolutely false. It is when one candidate is monstrously evil that a Catholic IS strictly, gravely obligated to vote for the person who has a chance of keeping the evil person out of office.)
Claiming that not voting at all is not formal cooperation in the evil Hillary will do is like standing next to a person having a heart attack, and doing nothing, and calling for nobody to help, and declaring, “I am innocent because I didn’t cause the heart attack.” Or similarly standing by when one COULD prevent a grave crime.
Refusing to vote for Trump so as not to “soil” oneself with Trump’s faults is like going into the voting booth and masturbating while yelling, “I am so pure! I am so pure! I am so pure!”
I see a rapidly fading possibility of stopping Hillary. A non-vote cannot contribute to stopping her; a vote for Trump might. A chance that is vanishingly remote but there it is. So --- to remove the personal element --- please understand that I am voting for Trump.
But I don't think it is morally wrong to abstain from voting, especially if you figure on the evidence that the chance of stopping Clinton by your vote is zero, but the chance of being personally corrupted by the vote is greater-than-zero. I know my vote is not going to swing the results at my polling place, North Side School; nor will it swing Washington County nor the State of Tennessee. This for a virtual certainty. It won't change the outcome. But it could change me by being another augmentation of my cynicism, callousness, and willingness to accept the mainstreaming of sleaze and re-norm my own life around the putrid cultural zeitgeist.
My own prudential judgment moves me to vote for Trump anyway. But if I thought it was corrupting to me I would not. I don't think abstaining from voting is a mortal sin if it is avoiding a near occasion of sin.
To quote you back at yourself, with modification,
"Voting for Trump as if it were morally obligation is like going into the voting booth, voting, and masturbating while yelling, “Salvation through sleaze! I am saving my soul! Everybody else is going to hell!"