If someone votes rationally, they are voting for a party (in general elections) rather than an individual. Policymaking is something that requires joint effort, and parties are groups committed to enacting policies.
If I believed in True Conservatism, I'd definitely be on the William Weld train. We need to get the word out to right-wingers that the only way we can have more tax cuts, more indiscriminate deregulation, more pollution, fewer brown people in our neighborhoods, and fewer interracial couples in television commercials is to vote for Weld. If he loses in the primary, we True Conservatives must, must, must write him in. The conservative movement can't survive another four years of Trump.