Daniel Larison comes up with the best formulation I’ve seen in a while to describe the difference between a realist and an ideologue:

The way to tell an ideologue from a realist, and the reason realists are not simply ideologues posing as something else, is that the ideologue will persist in a course of action long after it has failed and long after everyone knows it has failed because he thinks that his “values” demand it. Instead of “let justice be done, though the heavens fall,” the ideologue says, “I am right, and the world can go to hell if it doesn’t agree.” The ideologue is terrified of having to make adjustments and adapt to the world as it really is, because these adjustments reveal to the ideologue just how far removed from that reality he has become. The ideologue keeps redefining the justification for the policy, he keeps rewriting history to suit his own purposes, and he never accepts responsibility for the failure of his ideas, because he believes they have never been faithfully followed. For the realist, cutting one’s losses and reassessing the merits of a policy are always supposed to be possibilities, but for the ideologue the former is equivalent to surrender and the latter is inconceivable.