First, you have to vote. You have to either show up on election day or use an absentee ballot. Merely being lazy and not voting is NOT withholding your vote. You are a citizen of a democracy, even if the "democracy" is mostly myth. You have to vote.
Second, you have to give the candidate a warning. Either by letter, email, phone call, whatever, warn the candidate that they are not entitled to your vote simply because they are the Democratic candidate.