The left has always tried to silence dissent. That is one aspect of their desire not to listen to anything that does not agree with them, or to learn which has been a bit of a theme here recently, but it is also an aspect of the natural authoritarianism of the left.
If tyranny needs the silence of people of good conscience, how does tyranny gain that foothold? It does so by persuading people that speaking out is not an act of good conscience, it does so by shutting them up. It does so by crying ‘racism’ at every mention of immigration, at every criticism of or satire on a black person, at every arrest of Islamic suspects, or simply as a smear of an opponent. It does so by telling us to think of the children (in that link I am not commenting on the case, but the NSPCC reaction). It does so by saying ‘poverty’, and defining that word as broadly as possible. It does so by talking about workers’ rights, but never allowing rights to employers or customers (even when they are the same people). It does so by accusing opponents of its own flaws (astroturfing in the debate over Obama’s programme of legislation, financial interest in the climate debate).
The left has always tried to silence debate. It has had to more and more because it lost the rational debate; objective evidence of the 20th century is that socialism does not work, and never has.
All Tyranny needs to gain a foothold is that men of good conscience remain silent, so the left has to make sure such men remain silent, or are redefined as not having good conscience.