Political norms and the Supreme Court
Democrats are responding to a boringly normal exercise of constitutional authority by throwing a temper tantrum and threatening to smash our country's institution with radical changes to the system. It is the political equivalent of flipping over the Monopoly board when someone who already has Boardwalk and Park Place completes the set of three green properties on the last stretch before passing "GO."
There has been discussion about the historical precedents of approving or not approving nominees when the President and the Senate are of the same party or opposite parties. Both Republicans and Democrats have tried to be too clever about trying to make a high-minded principle about it, and both parties have come off badly as a result. The argument from Republicans ought to be really simple: The Republican majority wants to fill an empty seat with someone who shares the majority's judicial philosophy.
With that said, it is far from a historical anomaly for a President to nominate someone to fill an empty seat during an election year. In fact, incumbent Presidents always make a nomination during an election year. That includes Barack Obama. It is also not unusual for the Senate to vote to approve a nominee. Donald Trump is not "breaking our democracy" by nominating a replacement for Ruth Bader Ginsburg, and the U.S. Senate would not be "breaking our democracy" by approving that nominee.
What would be a radical shift is restructuring the Supreme Court or adding states to the union in order to game the system. There has been no serious effort to expand the court since Franklin Roosevelt attempted to do it after some decisions did not go his way. There have been 9 justices since 1869. That tradition has stood for over 150 years in a nation that is 244 years old - the majority of the nation's history.
The Democrats' fundamental flaw is treating the Supreme Court like a super-legislature instead of as an entity that interprets existing law as understood when those laws were written.
Packing the Supreme Court to cram through a "progressive" agenda is radically different from filling a vacant Supreme Court seat. If you think that our country is already bitterly divided over partisan lines, that will make it exponentially worse. At a time when we need to turn down the temperature of our political conflict, that would take us in exactly the wrong direction. A political party that openly promises to rig the system so they can win cannot be trusted with power, and Joe Biden does not deserve a single vote.