Think "Reagan Democrats." There are a lot of people who are conservative on social issues, but who often vote for Democrats on economic grounds. Many union members fit the profile. There's also a cultural element involved, as many African-Americans regard themselves as very conservative, but vote for the Democrat in most election. The same was long true of Jews, Catholics, Southerners, though the old New Deal ties have frayed over the years.
Liberal swing voters can be harder to find, but it may be a similar historical dynamic. In traditionally Republican states which have tilted Democratic in recent years, like Maine or Connecticut or Oregon, there are probably some people who'd define themselves as liberals who'd vote for Republicans.
The perception of Kerry as a moderate has something to do with his "free thinking phase of the early Nineties, when he questioned whether we could or should keep affirmative action forever. But Kerry never left the liberal reservation in terms of his voting record.
It als has a lot to do with liberals positioning of the candidate on the political spectrum to make him look more moderate than he is. Thinking of Kerry as a moderate allows liberals to see the Democrats as a more centrist organization than others portray it. But the perception cuts both ways: if Kerry is only a moderate, then what must fully fledged liberals believe?