The GOP lost the 2016 election, yet made up a rule that they couldn't even consider Obama's nominee. They didn't reject him. They refused to consider him, on made up basis that you can't seat a Supreme Court Justice in an election year.
Scalia died about 9 months before Obama's term ended. Yet, 45 days before Trump's election, this rule is now being violated. This is mob rule already. The GOP has already said that the party in power can do whatever it wants.
The Dems are just following through on that GOP expression of mob rule. Why should they follow rules the other side has no interest in following?