To be sure, Schlesinger’s partisanship too often influenced his scholarship. There came a point where he was unable to find an admiring word to say about any Republicans.
He was a liberal for sure (though far more moderate than he is often seen to be). And there may have been times when his writing was too influenced by his politics. But, his best writing is indesputably fantastic scholarship. And once a man dies, it is only the best of their work, and the most influential, that will endure. And it is that which we should focus upon.
Arthur Schlesinger was hardly the Liberal version of Ann Coulter.