Since Senator Kennedy introduced "borking" into the process.  After that, it's been anything goes.  In today's hyper-partisan environment it's not "your" seat because you have the Presidency;  it's only "your" seat if you have the Presidency and at least 50 Senators (assuming the nuclear option is invoked).  President Obama didn't in 2016.  President Trump does in 2017. 

I'm not suggesting that this is the way the system should work.  I'm stating that this is the way the system does work now.  We can have intense and unproductive discussions about who started it and how we got here.  The point is, neither party is likely unilaterally to disarm.  So if you don't like the current situation, elect more Democratic Senators.