At the tipping point, it is known that small incident can trigger dramatic societal shift. Getting early-warning signals for such changes are valuable to avoid detrimental outcomes such as riots or collapses of nations. However, it is notoriously hard to capture the processes of such transitions in the real-world. Here, we demonstrate the occurrence of a major shift in public opinion in the form of political support. Instead of simple swapping of ruling parties, we study the regime shift of a party popularity based on its attractiveness by examining the American presidential elections during 1980-2012. A single irreversible transition is detected in 1991. Once a transition happens, recovery to the original level of attractiveness does not bring popularity of the political party back. Remarkably, this transition is corroborated by tell-tale early-warning signature of critical slowing down. Our approach is applicable to shifts in public attitude within any social system.