He says he will serve out his term, just not run for leadership again.
The three leading contenders at the moment for the Senate Republican leadership are current whip John Thune, chair of the Senate Republican conference John Barrasso, and former whip John Cornyn.
As far as Ukraine aid is concerned, Thune and Cornyn support Ukraine aid. Barrasso is Trumpier and doesn't support Ukraine aid.
Thune would probably be closest to McConnell. McConnell has basically been trying to make him his successor.
Mitch McConnell obviously was extremely conservative and helped advance major setbacks to abortion rights. But his stepping down next November from leadership is another guardrail that won't be around during a potential Trump presidency.
Also a major loss for Ukraine support. But maybe he'll play an outsized role on Senate committees.