There has been a lot of talk about "white working-class voters" in this election season. I did a Google News search for that phrase and found 623 mentions, compared to 7 for "white middle class voters." That raises the question of why the candidates should be more interested in one type of voter rather than another--a vote is a vote. One possible reason is the size of the group--people used to talk a lot about the "farm vote," but that's faded as the number of farmers has declined. There are a lot white working-class people, but also a lot of white middle-class people, so that doesn't explain why the working class is getting all the attention now. Another possibility is the possibility of changing votes--a group that can be persuaded will get attention, while one that is pretty much fixed can be written off (or taken for granted). That raises the question of whether there are bigger "swings" among working-class voters. If so, then the focus on the (white) working class would make sense.
Before trying to answer, I should note that discussions in the media usually define "class" by income or education. My impression is that education is the more common standard. Sociologists normally define "class" by some combination of occupation and employment status, but most opinion polls stopped asking about occupation in the 1970s. Back then, a lot of people thought that class didn't matter any more, or that the working class was disappearing.
The American National Election Studies go back to 1952 and contain measures of income, occupation, and education. For income and occupation, there's no evidence that any group varies more than another, but for education there is: the "swing" among people without a high school degree is about 50% larger than the "swing" among people with a college degree. For example, say that the Democrats gain 4% vote share among college graduates than they did in the previous election--then they will probably gain about 6% among people without a college degree. (And about 5% among high school graduates). Candidates like Barry Goldwater and George McGovern did poorly among college graduates, but very poorly among less educated voters.
This pattern could be explained by saying that college graduates are more likely to have a strong ideological position which will keep them voting Democratic or Republican, while less educated voters are more likely to be influenced by everything else that's going on at the time. "Everything else" covers a lot of ground, but I looked at one thing that's easy to measure: incumbency. Being an incumbent president seems to help among all groups, but has a bigger impact among less educated voters. That also seems reasonable, if less educated voters tend to be less politically engaged and less ideological. This is based on a small sample (14 elections), but if the pattern holds, Romney's efforts to win over "working class" (less educated) voters won't pay off.