The SPD is known for being not left. He himself started out as stamocap, pretty much as far left as you could be in the SPD (now that's Die Like territory), but that was his youth. Now he's considered to belong to the right wing of the party but without clear association to any of the particular wings (there's at least two large left and two large right ones in total). Or let me put it this way: At least he's not a Seeheimer, the people who oversaw the turn of the SPD into New Labouresque bullshit and the dismantling of the welfare state, introduced a gigantic low-wage sector, etc. The left wing also doesn't actively oppose him, though with the SPD you never know whether it's the left wing simply lacking balls, again.