Here is a formula for deciding whether it is justifiable to decide whom to vote for based on a single issue:

Suppose something can be identified as the greatest evil of our time. If candidate X opposes the greatest evil of our time and candidate Y supports it, one should vote for candidate X in the absence of overwhelming factors.
Overwhelming factors could be of three kinds:
a) It is highly probable that a victory of X over Y would either not mitigate or exacerbate the greatest evil of our time.
b) It is highly probable that a victory of X over Y would lead to greater future evils.
c) The sum total of evils supported by X is demonstrably higher than the sum total of evils supported by Y.

I chose to focus on evils rather than goods because I find them more morally compelling, but you can think of evil as the negation or absence of a good. So, for instance, if a utopia is immanently possible but is being prevented, then that act of prevention might be considered the greatest evil of our time.

I don't want to argue about what the greatest evil of our time is, just the moral soundness of the above formula. So while examples of current day evils might be helpful to illustrate a point, I'd prefer not to focus too much on any particular issue, especially if it's politically controversial.