The discussion isn't abou what is "lawful" versus "unlawful", the discussion is who was doing what that could be considered "stacking" a result in their favour.
"Stacking" a result in one's favour, in this case the SCOTUS seat, would include not holding any nomination hearings for almost a year denying a sitting president their right to nominate a SCOTUS seat.
The "election year" line doesn't hold considering they allowed Trump to nominate a SCOTUS a couple of months before an election. More evidence of "stacking".
