Support for President Barack Obama and Mitt Romney among registered voters held steady in June, with the president leading his rival in a hypothetical match-up 47 percent to 44 percent, according to a new NBC News/Wall Street Journal poll. (In May, Obama led 47 to 43 percent.) But in 12 swing states where Romney is locked in an ad war Obama, the poll found Romney took a significant hit, NBC’s First Read team reported. Obama notched 50 percent support to Romney’s 42 percent in Colorado, Florida, Iowa, Michigan, Nevada, New Hampshire, New Mexico, North Carolina, Ohio, Pennsylvania, Virginia and Wisconsin. The new poll found that Romney’s unfavorability rating in those states jumped five points, compared to one point nationally. Romney may be more vulnerable to negative TV ads than Obama, for now, because he remains largely undefined, according to First Read. Romney is a “known name but an unknown person,” said Democratic NBC/WSJ co-pollster Peter Hart.