I think there is a semantic misunderstanding here. In the context of that writing, I think Possible=Legitimate; Necessary=Obligation.
When we say something is morally necessary, we mean we are obliged to do it. Thus, it is morally necessary to go to Mass every Sunday.
When we say something is morally possible or legitimate, we mean we can do it. But we are not obliged to. So I think necessary refers to the former.
But has anyone asked their Priest or Bishop? My estimate is 90% of Traditional Priests would tell Catholics to support and vote for President Trump, mainly for the pro-life, freedom of Catholic Nuns etc reasons.
I think it would be right to say there is a obligation to vote; and that obligation, for grave reasons, in certain circuмstances, could perhaps be dispensed from by a Bishop. But if you ask Priests, most will say to vote.
That's just my impression. Someone please check with a few Traditional Priests and Bishops and get back. I believe some here have the email of Bishop Fellay and the SSPX Bishops, Bp. Williamson and the Resistance Bishops etc.