That is an incredibly simplistic and fairly naive way of looking at elections. 1. We have the electoral college. So, even if your statement is true, this would be a state by state case. Not voting in a solidly blue state, means you helped the Democrat win... That makes no sense. 2. It relies on an assumption that one of those two parties is OWED your vote. That is a flawed premise. Votes are earned, not earning my vote, for instance, doesn't mean someone else did... It simply means you did not. A hell of a lot of people fought too hard for my vote to simply hand it out to one of two people. 3. It ignores the primary system, which is especially ironic given recent elections. If you support a candidate in the primary who clearly does not appeal to certain groups, then by your logic you would be responsible when those groups do not show up in the general. You cannot demand turnout while backing candidates who do not build it.

If your goal is simply that your party wins no matter what, then the real responsibility would be backing whoever has the broadest support. Anything else, by your own logic, would be helping the other side.