He quotes Alexander Hamilton in Federalist 65:
The prosecution of [impeachments], for this reason, will seldom fail to agitate the passions of the whole community, and to divide it into parties more or less friendly or inimical to the accused. In many cases it will connect itself with the pre-existing factions, and will enlist all their animosities, partialities, influences, and interests on one side or on the other; and in such cases there will always be the greatest danger that the decision will be regulated more by the comparative strength of the parties than by the real demonstrations of innocence or guilt.Since there is no conceivable doubt of guilt in this instance, says Wittes:
Despite this era of shredded norms and broken taboos, it is still verboten to state what is so obviously true: “I refuse to support Trump’s impeachment because, however merited it may be, I am a Republican and he is a Republican and the advantage of my party would be ill-served by his removal—which might also threaten my own prospects of reelection, which depend on voters who like the president more than they like me.”
The most difficult question, to me, is why voters in Republican leaning places like the Resident more than they like their Senator. After all their Senator votes for Supreme Court justices who will overturn Roe v. Wade. Their Senator has an A rating from the NRA and thumps the Bible. But their Senator generally speaks politely and in measured tones, doesn't threaten violence and hurl crude insults at enemies, doesn't claim to be entitled to sexually assault women, and doesn't overtly spew racism. Just a thought.