Nah, Greens and The Left are basically the same ideologically. All the differences are cultural.
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Well, that is contrary to the opinion of basically anyone knowledgeable about German politics.
I think this is a case where they can't see the forest for the trees. They are focused so much on detail they can't see the bigger picture that they are indeed basically the same. Maybe not the same, let me put it this way, the differences between the Greens and the Left are less than the differences between the left and right wings within other single political parties.
I fail to see how someone which annual birthday wishes to Fidel Castro and is nostalgic for DDR (a few old school PDS members) and someone like the minister-president of Bade-Wurtenberg (a green sounding like an old style christian-democrat) are similar.
Well in any merger the right section of "righter" party and left of the other one will be pretty far off. Now i don't enough to say about Die Linke and Green in general but in my short time living in Bremen their supporters were very similar and didn't seem to differ on many subjects (also lot of left wing people I met were undecided between the two) I don't know if it's a local thing in Bremen but they didn't seem that far of each other back then