Racism

Logical Form:

A has undesirable quality X.

B is associated with A in some way.

Therefore B has undesirable quality X.

Before I call fallacy, it would be worth specifying what 'supports' means; you need to elucidate. Person B could be supporting their racist behaviour, or some other aspect of their personality while either not commenting or even criticising their racism. On the other hand, they could also be a racist themselves, hence they are drawn to those policies - and thus, that person.

If there's no reason to belief that person B supports racism, then this is both an Association Fallacy and a Non Sequitur.

Like I said before though, you need to explain further what 'supports' means.

I assume that you mean that person A is overtly racist/has racist policies. In this circumstance then I would consider voting for, or giving any form of support to, that person would be an endorsement of such views and policy. 

Voting is intended to declare support for an elected candidate, therefore supporting their platform. Barring strategic voting and being forced to settle for a candidate whose policies you don't agree with due to a lack of better options, voting for an elected candidate means you at the very least support their platform over their opponents.

If someone is supporting a racist candidate in the sense of agreeing with their policies and wanting them implemented, then they're condoning their racist policies and/or refusing to hold them accountable for said racism. That's inherently racist.

I put it into an example for clarification:

The president is a racist. 

His supporters are ok with racism, because the president is racist. 

Seems like a bias, or tribalism.