It cannot be said too often: Leftism, whether it goes by “liberalism,” “progressivism,” “socialism,” or outright “communism,” is a religious faith. As with most faiths, believers regard apostates as the worst of evils:
The reasons are several, but paramount among them is this one: the Leftist regards his faith as a badge of intellectual and moral superiority. Anyone who voluntarily removes that badge has implicitly rejected that premise of superiority. Those who still wear it cannot escape that implication. It’s a thrust at their self-image they cannot endure.
Never underestimate the power of courtesy toward those with whom you disagree.