They don't necessarily have any relation whatsoever. Someone who employs marxist (little m) theory in their historical work is not necessarily a Marxist (big M) politically, though there are certainly historians who are. I think a lot of marxist historical theory is accepted broadly enough that you'd have trouble finding a historian today who doesn't have at least some marxist influence - marxist theory is the bedrock of social history, and most historians writing today will give at least some consideration to bottom-up history, the status of the common man, the role of class and economics, etc. Historians who are politically Marxist... I don't know any personally myself. One recent example that comes to mind is Christopher Hill (http://en.wikipedia.org/wiki/Christopher_Hill_(historian)), although I've only ever seen him brought up as a cautionary tale of what happens when you let dogmatic theory become more important than evidentiary analysis - he went to his grave a Soviet apologist.