As to artists who actually become politicians, Hitler, Ronald Reagan. Sonny Bono, Fred Thompson. I can't think of any artists who were progressive heads of state. John Wilkes Booth was a political "activist" of sorts.
[WS:] Vaclav Havel, a writer? http://en.wikipedia.org/wiki/V%C3%A1clav_Havel Not exactly a socialist, but fairly liberal.
Doctors seem to fare better cf. Salvatore Allende, or Che Guevara.
See also the list of modern heroes and killers at http://www.moreorless.au.com/ which offers a mixture of odd bedfellows (e.g. Churchill, Che Guevarra, Nelson Mandela, Joe Slovo and Mohammed Mossadegh on the side of the heroes, and Adolf Hitler, US presidents, or Saddam Hussein on the side of the killers).
Wojtek