Schaefer is, unfortunately, an individual example of the drawbacks of having an area completely dominated by one of the major political parties. Sure, Maryland has a Republican governor right now, but in many parts of this state, especially Baltimore city, the Democrats could run a trained monkey and still win the election. It’s the same for the Republicans in other parts of the country.
When one party has such dominance, its incentive to put up really good candidates decreases, and the internal party machine becomes more important than the wishes and the needs of the voters, because for the dominant party winning the primary and/or getting the support of party insiders is all you need to guarantee and election victory. And this system tends to favor those who already have built up powerful ties in the party hierarchy.
Of course, this insider’s or incumbent’s advantage is true in all politics, but it become particularly bad in areas where one party has an amost unassailable grip on elected offices.