If I wear a shirt with MEDICARE FOR ALL, GREEN NEW DEAL (or SUPPORT ACA and REJOIN PARIS ACCORDS) on it, that's clear partisan support. That's illegal. Thin blue line flags and "support our troops" are out of place and it's two-faced to pretend it isn't just a signal of partisan support. These are some of the major party issues.
Bad taste has nothing to do with it. If someone wears a SUPPORT THE CURRENT PRESIDENT shirt, that's support of Trump, not endorsement of the concept of government or something nonpartisan.