Applying the fundamental assumption of a pro-life position that all human life is sacred to all issues reveals that voting strictly for candidates who vote pro-life may guarantee a loss of life much greater than that lost through abortion. For instance, a pro-life candidate may also place economic issues over environmental issues. Lack of concern for the environment places the lives of existing and unborn individuals at risk, both through the short-term personal tragedy of serious health problems and the long-term societal tragedy of environmental destruction. Hence, voting on the single issue basis of pro-life may create other problems which also result in untimely loss of human life.
Let's consider another less heated case of voting on the basis of a single issue. It's fairly common today to acknowledge that using tobacco is harmful. Therefore, it seems quite logical to vote in a way that shuts down tobacco production and distribution immediately. Ignoring the practicality of such an attempt (think about prohibition, for example), consider the consequences of such a position. Jobs are lost all along the line all those involved in the production and distribution of tobacco will suffer because of the change in public policy. The loss of income will cause their families to suffer, their communities to suffer (through lower taxes), their schools to suffer, and so on. While very few will argue that we should encourage the production and consumption of tobacco, when we consider the effect of shutting down tobacco production immediately, it's not clear that this is a good thing.
A compassionate approach would consider the suffering of both users and producers of tobacco. For example, such consideration might provide those individuals in the tobacco industry with some amount of public financial support to move from tobacco production to another business. However, regardless of the particular solution, if it considers the producer as well as the consumer it is much more likely to be accepted by the producers, and hence more likely to succeed. When we villainize someone, they will not cooperate with us. When we recognize that they are not evil and assume that they are most likely people just like ourselves, we are much more apt to find a solution that is acceptable both to us and those who seem to be on the other side.
In general, voting on a single issue basis is almost certain to guarantee that you will be voting to bring about changes that will in fact lead to the result that you are so anxious to prevent. You can't ignore the whole of society while trying to fix a part. Fixing a part simply moves the symptom to another place and perhaps another time.