Manchin and his ilk should not define the center though. Joe Manchin is the most right-leaning Senator repping blue. He's not the center and shouldn't be treated as such in discourse because it'll do damage to how things get framed if Manchin is the barometer for the middle. Centrist isn't necessarily a bad word to me, UNLESS we're using it to describe people that would vote to confirm Kavanaugh and throw the rest of the party including leadership under the bus.
That said, I definitely agree that how we define candidates should come down to their platforms. I just feel that we should also expand the spectrum to fit in more policy options without the adroit spin from the right being to simply call anything remotely left leaning a step away from Fascist dictatorship (which they got away with throughout Obama's administration).