Here's an interesting statistic from a Associated Press article about Barack Obama's problems with white working class voters: Such voters made up 43% of the electorate in 2004. This means that a Democratic candidate who gets blown out in this demographic group is going to have a hard, if not impossible, time of winning the presidency.
The article notes that such voters have a more negative impression of Obama now than they did in February. This is from the article:
In an Associated Press-Yahoo News poll in April, 53 percent of whites who have not completed college viewed Obama unfavorably, up a dozen percentage points from November. During that period, the numbers viewing Clinton and Republican candidate John McCain negatively have stayed about even.
One obvious reason that Obama has trouble with such voters is race, but its not all about race. As the article makes clear, Clinton's campaign has more specific policy proposals aimed at such voters than Obama's campaign. The recent controversy over the gas tax is a good example.
There are a lot of reasons why the idea of a summer recess for the gas tax is a bad idea, but if you are struggling with higher fuel bills for your car, the prospect of lowering the cost per gallon is attractive. Its not that Clinton's policy is likely to be adopted, its that by coming up with the policy she is showing working class voters that she understands their concerns.
There is always tension in the Democratic Party between candidates backed by the intellectual wing of the Democratic Party and candidates backed by the working class wing of the Democratic Party. Candidates who can bridge that gap can win, but candidates who can't, don't. Kennedy in 1960; Johnson in 1964; Carter in 1976; and Clinton in both 1992 and 1996, were able to bridge the gap, and they won the presidency.
Right now the problem that Obama has is that there is mounting evidence that he will have a lot of problems bridging that gap. If he doesn't, and he is the nominee, Democrats will lose the race to McCain.