Sometimes, not long after you buy something of great value or cost, you start to wonder in your mind if you made the right decision. This might play in your mind so much so that you begin to regret your purchase or decision. This is buyer's remorse.
The expression is normally applied for feeling of regret after the decision. I am not familiar with American politics, so I can only guess that this must apply to the party members of the presidential candidate in question, since the general public haven't voted yet.