It's not okay behaviour on the Dad's part.

It wouldn't be okay behaviour if she was a Trump voter and he was left wing, either.

A few remarks over dinner "I'm concerned that candidate X has policy Y" are ok - but they should be said with respect for the other persons right to have a different opinion, and a willingness to drop it.

What the Dad is proposing is an *intervention* - which is only okay for seeking to address drug/alcohol problems; cult membership; untreated mental illness; gambling addictions; and abusive relationships.