Labour simply refused to acknowledge it. Until very recently, No 10 strategy, as defined by Morgan McSweeney, was built around neglecting, even insulting, progressive voters, and seeking to win back defections to Reform. Come the next general election, so the argument went, progressives would sheepishly have to back Labour, just as leftwing voters in France got behind Emmanuel Macron when push came to shove.
In what traditionally has been a safe Labour seat, there are two insurgent, challenger parties that are competitive. We are used to byelections where one outsider party is doing well, but here Reform UK and the Greens are both potential winners. This is further confirmation that the two-party system has completely broken down, and we are now in an era of multi-party politics.