So the real life Herman Mankiewicz was conservative and anti-union.

Great site. They gave Mank a reality score of 5.5/10.

The movie's attempt to tie Herman Mankiewicz to Democrat Upton Sinclair's loss in the 1934 California gubernatorial race is almost entirely fictional. Sinclair, who was a well-known author and prominent socialist, ran a campaign that challenged the state's business leaders. He had even written a book outlining his plan, titled I, Governor of California, and How I Ended Poverty: A True Story of the Future. Ultimately, Sinclair lost the race to Republican incumbent Frank Merriam. The movie somewhat downplays the fact that Mankiewicz was an outspoken conservative who staunchly opposed fascism. There's no evidence that he was a supporter of Upton Sinclair, nor does it make much sense that he would have backed him as a candidate.

he was an isolationist who thought the United States stood no chance against the German war machine, going so far as to declare himself “an ultra-Lindbergh.”

This movie sounds more like it's another Trumbo.

Surprising that Gary Oldman didn't object, since he's a bit of a conservative himself (didn't he criticise The Contender for being too liberal?)