But even as they have tried to expand the playing field of issues, Mr. Obama’s aides said they would not let up on their efforts to challenge the core of Mr. Romney’s campaign, his claim to be a better economic manager. …

To drive home that point over the past few weeks, Mr. Obama has gone on an advertising binge, spending more than $12 million during a single week in mid-June, according to a Republican group that monitors advertising spending. …

In the most recent NBC News/Wall Street Journal poll, 33 percent of respondents in swing states said that hearing or reading about Mr. Romney’s business record had made them view him more negatively, as opposed to 18 percent who said it made them view him more favorably.