Googling a little, he was a dem:

>Having reportedly voted for Joe Biden in 2020, Musk even voiced his pro-Dems alignment in 2022 when he posted on X, formerly Twitter, that he had “strongly supported Obama for President” in 2007.

I think he turned after Tesla was snubbed at Biden's 2021 EV summit because although it was the US's largest EV maker it wasn't unionized and Biden was in with the unions.

I can sort of see that being annoying.