Why? Because you have abandoned your calling to be a spokesman for God, his messenger, and become instead a spokesman for a political party. You have moved away from your calling as a Pastor/Teacher to become a politician’s surrogate.
By so doing, regardless of your political viewpoint, you have turned your back on the Word of God which calls you not to endorsements of politicians but to proclamation of the Word of God, which stands against all human systems and which therefore demands that the representatives of his Word do the same.
Your task isn’t to act as cheerleader for fallen human politicos. And if you think it is, and you must or you wouldn’t allow yourself to be co-opted by human ideology, then you most assuredly have cut yourself off from the Word which you are commanded to proclaim. Indeed, you don’t even know that Word sufficiently well to know your own task: you surely aren’t in a position, then, to tell others what it says.
So, again, if you stood in your pulpit yesterday and told your congregation that you were voting for so and so and endorsing so and so for the office of the Presidency, quit today. You are no longer a Pastor.