I still think there is some misunderstanding so I’ll try to be more precise. It may be good policy to pass laws and regulations prohibiting ppartisan bias in government. But if it were in fact unconstitutional, then no laws would be needed to prevent it.
Your citation of laws, not provisions of the Constitution, is evidence against your position on the question of what the Constitution allows. You say:
On the contrary, there is plenty of legislation on the matter. Let’s start with the Pendleton Civil Service Reform Act of 1883. Then let’s move to the Hatch Act of 1939. Moreover, use of government resources for political activity is explicitly forbidden by the Department of the Interior, the Department of Justice, the House of Representatives, and the Department of Defense, among others.
So, before Civil Service it was perfectly Constitutional and legal to fire and hire most civil servants on partisan grounds. We needed a law to stop that.
Of course it is Constitutional to pass laws and regulations limiting partisan influence in government. When the Constitution was passed, the development of political parties was not anticipated and there is nothing in the document specifically condoning or constraining such parties.
Finally, to support the assertion you are making as about Constitutionality, you need to find a case in which the Supreme Court tossed out a law or regulation, otherwise Constitutional, on grounds that it was motivated by partisan advantage. I don’t think such a case exists. I think you have invented a Constitutional principle that just does not exist, no matter how much you think it should.