William Hazlitt explained the nature of it in his 1820 essay, "On the Spirit of Partisanship."

Conservatives and liberals play the game of politics differently, Hazlitt wrote, because they have different motivations. Liberals are motivated by principles and tend to believe that personal honor can be spared in political combat. They may, in fact, become vain about their highmindedness. Hazlitt condemns the mildness as a mistake, both in moral reasoning and in political strategy. "They betray the cause by not defending it as it is attacked, tooth and nail, might and main, without exception and without remorse."

The conservatives, on the other hand, start with a personal interest in the conflict. Not wishing to lose their hold on power, they are fiercer. "We"---i.e., the liberals, or the "popular cause," in Hazlitt's terminology---"stand in awe of their threats, because in the absence of passion we are tender of our persons.

They beat us in courage and in intellect, because we have nothing but the common good to sharpen our faculties or goad our will; they have no less an alternative in view than to be uncontrolled masters of mankind or to be hurled from high---

"To grinning scorn a sacrifice,
And endless infamy!"

They do not celebrate the triumphs of their enemies as their own: it is with them a more feeling disputation. They never give an inch of ground that they can keep; they keep all that they can get; they make no concessions that can redound to their own discredit; they assume all that makes for them; if they pause it is to gain time; if they offer terms it is to break them: they keep no faith with enemies: if you relax in your exertions, they persevere the more: if you make new efforts, they redouble theirs. While they give no quarter, you stand upon mere ceremony. While they are cutting your throat, or putting the gag in your mouth, you talk of nothing but liberality, freedom of inquiry, and douce humanité. Their object is to destroy you, your object is to spare them---to treat them according to your own fancied dignity. They have sense and spirit enough to take all advantages that will further their cause: you have pedantry and pusillanimity enough to undertake the defence of yours, in order to defeat it. It is the difference between the efficient and the inefficient; and this again resolves itself into the difference between a speculative proposition and a practical interest.

It is not fair play, and Hazlitt thinks that liberals who decline to fight fire with fire are fools. "It might as well be said that a man has a right to knock me on the head on the highway, and that I am only to use mildness and persuasion in return, as best suited to the justice of my cause; as that I am not to retaliate and make reprisal on the common enemies of mankind in their own style and mode of execution."