Poor decision making knows no ideology. I often hear the same mistake from liberals and conservatives on abortion, liberals and libertarians on civil liberties and everyone on economics. It seems the entire political spectrum has been united in the error of single issue voting. Whether it is through so called dealbreakers, single issues which are disqualifying positions, or the positive version which centers the candidate with the best policy on a single issue many seem to think “I should (or shouldn’t) vote for candidate A based solely on issue X” is a reasonable argument. However, making a decision on the basis of a single issue, without even considering other issues, is generally only rational if no other issues affect outcomes but in voting this is clearly not the case.
If we remove single issue decision making from the political arena this would be evident. Some people like to imagine that they purchase a vehicle based solely on which is the safest. However, when you imagine the potential differences in safety for a vehicle, from a tank to a moped, you realize that a reasonable decision must include some tradeoffs. In fact to disregard all tradeoffs would mean that you would be, in theory, willing to pay infinitely more money for any improvement in safety no matter how small. Not only is this a poor model for how people actually behave but it is also a reminder that the magnitude of each consideration is important. You may value safety more than style or price but not infinitely more so.
To return this to politics that would mean a single issue voter, or one with dealbreakers, would be willing to forgo improvements in outcomes on all other issues, no matter how large their impact, for the sake of any improvement on their chosen issue, no matter how small. In practice this would mean the single issue voter who says the only relevant issue is economics would be willing to vote for a candidate with unfavorable policies on human rights, foreign policy, social issues, etc. for any marginal improvement in economic policy. Obviously this is not a path to overall goal fulfillment as a voter’s interests could well be met by selecting the candidate who is second or third on their most important issue but who is better overall (to say nothing of the chance of that candidate actually winning).
Moreover this highlights that separating issues in politics is really more of a convenient tool than an accurate description of reality. In reality a candidate’s policy on, say, war, has an influence on the economy, and vice versa, so treating each issue separately without regard to the overall outcome makes even less sense. Granted, outside of voting, there will be times when single variable decision making will generally result in the best decision, and situations where the effort required for a more thorough analysis is not worth the improved return, but in important decisions one should at least examine all of the attributes of the available choices. Voting for or against a candidate based solely on a single issue clearly fails to consider all relevant goals and as a result is likely a very poor guide to achieving the best overall outcome.