CHARLOTTE, N.C. — A day after fumbling a predictable and straightforward question posed by Mitt Romney last week — are Americans better off than they were four years ago — the Obama campaign provided a response on Monday that it said would be hammered home during the Democratic convention here this week: “Absolutely.”

The focus on the campaign’s handling of the question, after halting and contradictory responses from Democrats on Sunday, complicated the White House’s effort to begin striking a set of themes the president intends to highlight here and carry through the general election.

That effort starts with an argument that Mr. Romney, the Republican nominee, would raise taxes on the middle class while cutting them for the wealthy. It seeks to pitch forward to the next four years the case that Mr. Obama and his allies have made over the spring and summer — that Mr. Romney’s business career showed him intent on profit even at the expense of workers and that his wealth has given him tax advantages not enjoyed by regular people.

“The problem is everybody’s already seen his economic playbook,” Mr. Obama said at a campaign stop in Ohio before a Labor Day audience largely consisting of United Auto Workers union members. “On first down he hikes taxes by nearly $2,000 on the average family with kids in order to pay for a massive tax cut for multimillionaires.”