If literally the only thing standing in the way of sunshine and roses in this picture is that the Republicans are in charge, why is it so necessary for the Democrats to put them in charge?

The Democrats could just let the post office be privatised then run their own, on the side, out of to the Democrat HQ. It'd give them an electoral edge delivering pamphlets too.

The reason they can't do this is because the post office, fundamentally and inescapably, doesn't generate enough revenue to justify running it the way they want to. The Democrat voters don't want to bear the cost of the service they want. And the Republicans have noticed this.