Funny things happen to the human body above 26,000 feet (8,000 metres). Brain cells die. Blood turns to thick red custard. Vessels in the eye spontaneously burst. Brain swelling can lead to coma or worse. Mountaineers call it “the death zone”, and with good reason. In British politics the death zone is less visceral, but no less serious. Any party that spends too long in the 20s or below in the polls is in deep trouble. Usually only one major party is unlucky enough to be so disliked. Today, they all are.