What was the significance of the subplot involving 1934 California gubernatorial election in Mank?

Question

Mank (2020) film discuss about film about screenwriter Herman J. Mankiewicz and his  development of the screenplay for Citizen Kane (1941). 

I didn't understand the purpose of subplot "1934 California gubernatorial election" in this film.

Here is the movie screenshot of voting board of 1934 California gubernatorial election at an election night watch party at the Trocadero Nightclub.

Answer 1

Held in the midst of the Great Depression, the 1934 election was amongst the most controversial in the state's political history.¹

Negative campaigning funded by the film industry was used against Sinclair to favor the Merriam campaign.²

In 1934, Herman and Joseph begin working at MGM under Mayer. Studio executives, including Irving Thalberg, actively work against Sinclair's gubernatorial campaign. The studio produces propaganda films for a smear campaign, funded by Hearst, against Sinclair.³

It's another important part of the animosity between Hearst and Mank. If you remember that "during Upton Sinclair's race for governor of California in 1934, Mank refused the MGM chief Louis B. Mayer's order to hand over a donation to Sinclair's Republican opponent", then the election is clearly another page of their not-so-friendly story.

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^{1. 2. 3. from Wikipedia}