Since the 80s political scientists have pointed to the 'myth of the independent voter'. It's been covered in most basic college poli-sci texts since that time.
Most data seems to indicate that of the electorate, only 10-11% of self proclaimed independents are truly independent. This is from Pew, Gallup, ANES studies/polls. Most independents are 'leaners' that fall in along dem/GOP party lines, rivalling the support of self proclaimed partisans.
The number of people who are not registering as a member of a party is increasing, but truly independent voters are much rarer.
Partisan shares the same root word as party for a reason. You have to belong to or support a party to be a partisan.
Yes, the etemology of the word is not under dispute, certainly not by me.
But people can and do call themselves independent and still consistently support a particular party, or specific ideology that aligns with a party.
Calling yourself something and actually being that thing in practice and principle are different.
All God's children are not beautiful. Most of God's children are, in fact, barely presentable.
I never met anyone who didn't have a very smart child. What happens to these children, you wonder, when they reach adulthood?