Yanukovych's decision to withdraw from the EU Association Agreement in 2013 was a significant and controversial political move for sure. The decision was more a matter of executive policy rather than a direct constitutional issue though. Yanukovych's authority to make such foreign policy decisions was within his presidential powers, but the lack of consultation with other branches of government or the public raised legit questions about the democratic appropriateness of the move. I guess it means he may have lost the next election if the opposition had EU Association as a cornerstone of their policy but we will never know because of the coup.
