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.
I think it's obvious that most people calling themselves independent lean one way or the other ideologically. But that's besides the point.
The distinction isn't between independents vs people belonging to a political party but by people that defend their party no matter what, and hate the other side no matter what. That's what's usually meant by partisan people.
You can be a registered democrat/republican but not be hyper partisan. You can be a critical thinker and belong to a party, it's just that some people let their identity become wedded to the party to the point where it's almost impossible to be objective.
