An attitude and an opinion. Same thing, right?
Nope. At least not to social scientists who study this stuff.
Getting a little PhDweebish, but opinions are observable manifestations of an attitude. An attitude is a durable orientation toward some object, while an opinion is more of a visible expression of an attitude -- kinda like answering a survey question.
Why does this matter? Studying this stuff is in part how we define concepts, in part how we measure them, and in part how we examine the relationships among them. It's great fun, but you have to be careful in meaning what you mean. I may have an attitude about an object (or person, or issue, or whatever) but I've never expressed it, never really given it much thought, up until someone asks me or I am forced in some way to access my attitude. So they're not interchangeable, at least not in social science, though in journalism we do exactly that all the time.
Why am I going on about this? Well, values and beliefs get mixed up with attitudes and opinions and -- yes -- political knowledge (you knew I was going there, right?). Lemme borrow from one text: "For example, one's attitude toward the welfare system does not flow directly from relevant core values like individualism or equality, but from the linkage of these values to what is known or believed to be true about the specific program and the environment in which it will operate" (Delli Carpini & Keeter, 1996).
In other words, knowledge gets in there too -- either factual knowledge or, and this is important, perceived knowledge. What we think we know.
Throw in our tendency to selectively expose ourselves to information we prefer, or to misremember what we read or hear and fit it into our own predispositions -- well, that explains how some people go off on tangents about presidential candidates, how they're sure candidate x is this or candidate y is that. Values, beliefs, attitudes, knowledge (real or otherwise), all tied eventually into an opinion, one perhaps yelled out at a rally and people nearby shake their heads in agreement, even if what was yelled was absolute bullshit.
Doesn't matter. It's all a mess up there in the black box, a nest of inconsistencies, facts, values, beliefs, and the end result through some bizarre algebraic formula are the opinions expressed to pollsters. It's a wonder we can walk and chew gum at the same time, or do both while answering a survey question.