Third rail of politics

Third rail of politics is an expression that is used in politics to talk about an issue or topic that's so sensitive, that politicians avoid it because it could cause a lot of problems. For example, if a politician talks about something like gun control, they may stay away from it because it could make some people mad or they could lose votes because some people disagree with them. The expression comes from the third rail on a train track - the part that has electricity running through it. If you touch the third rail, it could cause you a lot of harm, similarly talking about certain topics could cause harm for a politician.