If you want to understand what the battle over Mitt Romney’s Bain years is really about, this chart gets to the heart of it.

Drawn up by the labor organization Working America a few months ago, when the war over Occupy Wall Street was in full swing, it’s even more relevant now. It shows how dramatically the white working class vote in five key swing states has swung in recent elections.

The chart demonstrates that in 2008, Obama won this key demographic by double-digit margins in Florida, Ohio, Oregon, Pennsylvania, and Wisconsin — and went on to lose in four and tie in one only two years later:

A second chart, this one drawn up by The New York Times, also tells the story. It finds that nationally, white working class voters — defined as having no college education and making between $30,000 and $75,000 a year — were key to the last few presidential elections, and continue to swing back and forth

In 2004, Kerry only took 32 percent of this demographic; but in 2008, Obama surged up to 46 percent of it — a far higher share than Dems usually command — enroute to victory. Only two years later the Dem share of that vote sagged down to 34 percent as Republicans took back the House.

The Dem attacks on Romney’s Bain years are all about limiting Dem losses among this key consistuency. Because of ongoing demographic shifts, many think that Obama does not necessarily have to duplicate his 2008 showing with this group in order to reach 270 electoral votes — he merely has to limit slippage among them. According to a recent study by the Center for American Progress, if Obama can win back enough white college graduate voters to get back to 2008 levels among that group, Obama can still win reelection if he loses blue collar whites at the disastrous levels Dems suffered in 2010. If Obama can hold his losses among both to around the levels Kerry suffered in 2004, he can still win, given a solid performance among minority voters, who have been gaining as a share of the electorate.

The ongoing Dem attack on Romney’s Bain years — which will figure heavily in the general election — could help decide whether Obama can hold blue collar white voters in the numbers he needs. Given that these voters have born the brunt of the economic meltdown, it’s interesting that the GOP is set to nominate a candidate who got very rich partly off the sort of layoffs that helped decimate their communities; pays a lower tax rate than many middle class taxpayers because of investments but won’t release his tax returns; and refuses to say whether concerns about inequality and Wall Street excess are rooted in anything but “envy.”