He could potentially win Michigan based on this, so you’re probably right in that they did some polls and found this to be a domestically “good” move.
The auto industry is huge in Detroit. If Biden didn’t do this, I could imagine a situation where these budget-friendly Chinese vehicles actually scare the Big Three Auto Companies and the UAW into backing Trump, which would be a disaster.
I believe a defense for the tariffs is that the Chinese government is heavily subsidizing them, but I do not know how true that is.