I'm going to push back against your narrative, a little bit, with the following questions:

1.) Is there any compelling reason why every citizen should proactively self-identify as either a Democrat or a Republican?

2.) What adjective/identifier would you assign to somebody who holds a simultaneous mixture of liberal/progressive ideas, moderate ideas, and conservative/libertarian ideas?

3.) What is your response to those (such as myself) who would say that centrism is more about finding rational, well-balanced solutions to problems -- rather than merely "splitting the difference" between two extremes for the sake of show? (and, in many cases, some of those solutions could be ones with which self-identified progressives are very pleased)