If you hear someone use that phrase, you know that they overwhelmingly vote for candidates of a single party. This is just something people say and it goes to why the idea that independent voters are important is a myth.
Personally, I hear that phrase most often from people in my extended family who know that I’m a yellow dog Democrat and a former political operative for Democratic candidates and don’t want to say, flat out, they are hard core Republicans or, maybe, and this will be important, don’t want to believe they are party line voters. But when I ask, after hearing that phrase, for the name of the last Democrat they voted for or for the names of the last six presidential candidates they voted for… well, you get the picture.
The overwhelming majority of people who declare themselves independent actually vote almost exclusively for candidates of a single party at the same rate as people who actually declare a party. The majority of people declare themselves independent for a couple of reasons, both related to pride. Being politically independent fits in with an image from American mythology. It is also a way to distance yourself from a political party whose name is in the mud.
Mitt Romney won ‘independent’ voters because the GOP brand was not great, so a lot of folks who might previously have declared themselves Republican, claimed to be independent. But they still voted Republican.
Here’s another prediction. The ‘independent’ vote will be close in November. Trump will either win or come very, very close in the independent share of the vote because a lot of… shall we call them yellow dog Republicans? …will still vote straight ticket GOP, but will be embarrassed of their true party affiliation because, well, Trump is freaking embarrassing.