Bottom line is Trump isnt going to nominate anyone the left will vote to confirm and visa versa. So therefore the nonimee should be the most conservative person Mitch thinks he can get confirmed.
No downside to doing it before the election. Trump wins and we got another conservative long term on the court. Trump loses and we got another conservative on the court long term. The risky move is to wait as there is no guarantee both Trump with win and the Senate will remain in Republican hands for four more years.
