The battle to define Mitt Romney as emblematic of everything that’s wrong with our economy and tax code is largely about winning back a core Democratic constituency that Obama badly needs in 2012: Blue collar whites.

Some conservatives worry Romney may be the wrong candidate to hold on to the massive 2010 gains Republicans made with these voters, which is why they’re nervous about the attacks on Romney's Bain years, his privileged tax rate, and his propensity to say things that make him sound like an out of touch fop.

Today the Post has released some new polling that contains a remarkable finding: Romney’s unfavorability rating has spiked massively among these voters.

Among overall Americans, Romney’s favorability rating — a measure of broad acceptability — has dropped eight points since earlier this month, to 31 percent. His unfavorable rating has jumped 15 points, to 49 percent. That’s a net swing of 23 points. His negative rating now tops 50 among independents.

But the most interesting finding is among whites with incomes of under $50,000: His negative numbers among them have jumped 20 points, from 29 percent to 49 percent.

This is important. In 2008, one key to Obama’s victory was that he pulled in 46 percent of whites with incomes between $30,000 and $75,000, an uncharacteristically high number for a Dem presidential nominee. Two years later, Democrats slumped to 34 percent among these voters amid their midterm shellacking. In key swing states, these losses were even more pronounced.

If Romney proves unpopular among these voters, it’s not impossible that Dems could win them back in the numbers they need next year. It’s true that Obama’s numbers among them remain worse than Romney’s; his negative rating is 56 percent. But according to a recent Center for American Progress study, Obama only has to limit his losses among this demographic in order to win reelection, provided he does reasonably well with upscale, college-educated whites — and today’s Post poll finds he’s rising with that group.

Romney has taken an unusually rough battering in recent days, and he has plenty of time to turn things around. But the spike in negative views of Romney among blue collar whites suggests the possibility that the assault on his wealth, privilege, low tax rates and generally out of touch persona could be resonating with them, and is possibly beginning to define Romney among them. One imagines Republicans who are gauging Romney’s electability in a general election — and were already inclined to doubt his strength among these voters — might find this somewhat worrisome.