leftist:
(also ‘left-winger‘ or ‘the left‘; plural: ‘leftists‘; the opposite word in meaning to this word: ‘rightist‘)
a member or supporter of the “left” group in politics,

The Obamas not leftists for using left hands (right hand to be used) during pledge (image photoshopped by the owner!)
They say Gordon Brown, the British Prime Minister, is a better leftist than Tony Blair, the former British prime minister.
Some political analysts believe that the leftists in some governments criticise other systems without ever explaining what they are and how they differ from the leftists’ own ideologies.
Some people believe that, in American politics, leftists are liberal, i.e. leftism and liberalism are the same, but some others contradict this theory and argue that they need definite explanation on how they are the same.
