One of the many dirty tricks that the Republicans use to remain in power is to pass laws that selectively disenfranchise minorities and other likely Democratic voters. So I don't think this is disingenuous at all. The fact that fewer people vote is in no small measure because of Republican dirty tricks [1].
I deliberately chose that phraseology to be ambiguous. But what is not ambiguous is that the Republicans engage in dirty tricks (gerrymandering [1], stripping the governor of powers after an election where a Democrat is elected [2], passing laws to disenfranchise demographic minorities [3], running candidates with blatant conflicts of interest [4], and just flat-out breaking the law [5]) far more often than Democrats do.
I think it is a valid observation that statistically, the Republican Party represents fewer citizens per politician than the Democratic Party. This is partly due to geography and the deliberate choices of Senate representation, and partly due to the skillful manipulation of state politics by the Republican Party. The Democrats collectively forgot for a while that politics starts at the local level, unsexy as it seems compared to, say, POTUS, and the Republicans cleaned their clock.
Of course, currently the Democratic Party relies heavily on actual racial/gender minorities, but if they are smart they understand how fleeting that association can be, there is no guarantee it will remain durable over time. Especially as the number of older white male voters dwindles through natural attrition, it is pretty likely that the demographics of the parties are going to evolve.
Eventually white people are going to figure out they want healthcare and black people are going to figure out they want guns and all hell will break loose.
[1] https://www.nytimes.com/2018/11/03/us/politics/voting-suppre...