Wiktionary
abstentionist
n. A practitioner or advocate of not getting involved politically. (First attested in the late 19th century.)(R:SOED5: page=9)
Usage examples of "abstentionist".
The City of God was an abstentionist sect, but the people were desperate for any kind of nourishment and the effect of alcohol on empty stomachs would not have been a good thing.
All shades of political opinion - Abstentionists, Moderates, the Comitato Patriottico - were represented.