(AP) - On the eve of the Democratic National Convention, President Barack Obama declared that Republican rival Mitt Romney should be penalized for "unnecessary roughness" on the middle class and accused him in a ringing labor Day speech of backing higher taxes for millions after opposing the 2009 auto industry bailout.

"I've got one piece of advice for you about the Romney-Ryan game plan: Punt it away. It won't work. It won't win the game," Obama said, blending sports-themed remarks with economic barbs before a cheering crowd in the nation's industrial heartland.

He backed up his rally comments with a new television commercial that says Romney doesn't understand the "heavy load" the middle class is carrying yet wants to give himself a big new tax break. It's the president's first new ad since last week's Republican National Convention, a reminder that he and his allies have been outspent by millions in the ad wars over the past several weeks.

His sports comments in Toledo, Ohio, amounted to a rebuttal to Romney's weekend appeal to voters to fire the current coach - Obama - and install the Republicans instead at the controls of an economy sputtering along with 8.3 percent unemployment.

The president headed to hurricane-damaged Louisiana late in the day as he slowly made his way toward the Democrats' convention city. First lady Michelle Obama was already there, and made a quick trip to check out the stage at the Time Warner Cable Arena where she will speak on Tuesday night.

A few blocks from the hall where Democratic delegates will gather on Tuesday, union members staged a Labor Day march through downtown. Though supporting Obama, they also expressed frustration that he and the Democrats chose to hold their convention in a state that bans collective bargaining for teachers and other public employees.

There was disagreement among the ranks of the marchers. "I understand their frustration ... but do they really think they're going to be better off with Romney?" asked Phil Wheeler, 70, a delegate from Connecticut and a retired member of United Auto Workers Local 376 in Hartford.

Democrats chose the state to underscore their determination to contest it in the fall campaign. Obama carried North Carolina by 14,000 votes in 2008, but he faces a tough challenge this time given statewide unemployment of 9.6 percent in the most recent tabulation.