For all of his many faults, McConnell considers himself a patriot and wants the system to continue to function as a democracy. He’s never been onboard the Trump train, and definitely isn’t a fan of the move of the Republican party towards fascism…the fact that he’s one of the people responsible for that path probably isn’t something he realizes. And, he probably does see the possibility of Democrats cheating the system to install their own candidate as president, especially as demographics shift, and he wants to keep Republicans in power as long as possible.
Put all that together, and it’s not really a surprise he’d support this…I’m sure he it looking at it as a way to guarantee no interference when Republicans win the White House, because he’s focused on Republican power, but it’s still also a good thing for the nation. We’ll see if there is follow through though, it’s always possible he’s planning to torpedo this because he’s secretly onboard the Trump train but has hidden it until now.