U.S. President Barack Obama consoled victims of Hurricane Isaac along the Gulf Coast on Monday and stoked the enthusiasm of union voters in the industrial heartland, blending a hard political sell with a softer show of sympathy on the eve of the Democratic National Convention.

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U.S. President Barack Obama walks past debris on the sidewalks as he tours the Bridgewood neighbourhood in LaPlace, La. (Pablo Martinez Monsivais/Associated Press)

At times like these "nobody's a Democrat or a Republican, we're all just Americans looking out for one another," the president said after inspecting damage inflicted by the storm and hugging some of its victims. He was flanked by local and state officials of both parties as he spoke.

There was nothing nonpartisan about his earlier appearance in Toledo, Ohio. There, the president said Republican challenger Mitt Romney should be penalized for "unnecessary roughness" on the middle class and accused him in a ringing Labour Day speech of backing higher taxes for millions after opposing the 2009 auto industry bailout.

Obama's trip to La Place, La., was a televised interlude in the rough and tumble of the political campaign, four days after Romney accepted his party's presidential nomination at the GOP Convention in Tampa, Fla., and three days before the president is nominated by Democratic delegates in Charlotte.

Arrives in North Carolina later in the week

Unlike Obama, Romney made no mention of federal aid in his trip to Louisiana, which was designed to demonstrate his own concern for victims of the storm.

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U.S. President Barack Obama, right, greets a local resident as he tours the Bridgewood neighbourhood in LaPlace, La. (Pablo Martinez Monsivais/Associated Press)

First Lady Michelle Obama was already in the Democratic convention city as her husband spent his day blending the work of president and candidate. Obama doesn't arrive in North Carolina until later in the week, after concluding a slow circuit of campaign stops in battleground states and the trip to Louisiana.

In the flooded neighbourhood, he said he had promised local residents "we're going to make sure at the federal level, we are getting on the case very quickly about figuring out what exactly happened here, what can we do to make sure that it doesn't happen again and expediting some of the decisions that may need to be made to ensure that we have the infrastructure in place to protect people's property and to protect people's lives."

The federal government spent more than $10 billion to strengthen the levee system around New Orleans after the devastation of Hurricane Katrina seven years ago.

Obama noted that last week's flooding was in a different region, leaving open the question of what the government might do to prevent a recurrence.

A few hundred kilometres away in Charlotte, the conversion of the Time Warner Cable Arena into a political convention hall was nearly complete.

A few blocks from the hall where Democratic delegates will gather on Tuesday, union members staged a Labour 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 per cent in the most recent tabulation.