Complex sentence: The unanimous opposition to Stuttgart 21 has led to "unprecedented popularity" for the Alliance' 90/The Greens in Baden-Württemberg, and has led the party into the role of presumably being the senior partner in a new coalition government with the Social Democratic Party.
Split into simpler sentences: The unanimous opposition to Stuttgart 21 has led to "unprecedented popularity" for the Alliance' 90/The Greens in Baden-Württemberg. The "unprecedented popularity" has led to rise of the Green Party, presumably being the senior partner in a new coalition government with the Social Democratic Party.