Bunker mentality is a slang phrase for a phenomenon that occurs when an individual or group stops taking into account new, pertinent information and begins viewing outsiders as enemies due to an isolation resulting from being under attack. Political campaigns and figures are often accused of having this mentality, particularly when a leader, administration or party has become unpopular or is in some sort of trouble.[1]
The phrase is taken from an analogy to soldiers that have taken shelter in a bunker while under siege from enemies as well as Adolf Hitler's mental state during his last days inside the Chancellery bunker in Berlin.