Tropical Storm Isaac churned through the Gulf of Mexico on Monday, threatening to batter the nation’s Gulf Coast on the seventh anniversary of Hurricane Katrina and casting a shadow over this week’s Republican National Convention in Tampa.

The storm’s maximum sustained winds rose to 70 mph late Monday afternoon, just 4 mph below hurricane status. Forecasters expected it to develop into a sprawling Category 1 hurricane and make landfall on the Louisiana or Mississippi coast by early Wednesday.

Isaac could bring sizable storm surges and massive amounts of rain, presenting the most stern test yet for New Orleans’s rebuilt levees, which are designed to endure all but the most catastrophic storms. The storm is also likely to test the federal government’s ability to respond to a natural disaster in the region seven years after the Bush administration fumbled the task in the wake of Katrina.

For Republicans in Tampa, the coming storm presents another kind of challenge. With Isaac bearing down on the Gulf Coast, party leaders have been left to walk a delicate political tightrope: trying to bash President Obama’s handling of the economy and spotlight the GOP nominee, Mitt Romney, while potentially sharing broadcast time with scenes of hurricane devastation and the Obama administration’s response to it.

Officials on Monday urged those in Isaac’s wide-ranging path to take the storm seriously. Its huge size and sluggish forward progress — it is forecast to slow further as it comes ashore — could mean storm surges of six to 12 feet in low-lying coastal areas and heavy rains.

“These water hazards — the storm surge and the inland flooding — are things that sometimes people forget,” National Hurricane Center Director Rick Knabb told reporters Monday. “They consider tropical storms and hurricanes as just windstorms, and they are far more than that.”

Federal Emergency Management Agency Administrator Craig Fugate urged residents to flee if asked. “When those evacuation orders are issued, we need people to heed them and move to higher ground,” he said. “Don’t wait.”

That sense of urgency had yet to pervade New Orleans on Monday, which cooked in 90-plus-degree heat beneath clear blue skies. City officials warned that Isaac could be a dangerous storm but stopped short of mandatory evacuations.

New Orleans Mayor Mitch Landrieu sought to reassure residents about the threat posed by Isaac, taking to a podium in City Hall to boast that “there’s nothing this storm will bring us that we’re not capable of handling.”

He touted $10 billion of levee improvements since Hurricane Katrina and proclaimed the city “battle-ready.” A Category 3 hurricane — with winds more than 110 mph — might have prompted an evacuation, Landrieu said, but Isaac was looking likely to remain a Category 1.

Isaac’s approach did force the cancellation of events staged each year to remember those killed by Katrina. Also, prison superintendents worked through the night to transfer more than 1,000 inmates from vulnerable jails in the city.

But much of the city moved at the same languid pace as on any other day. Tourists strolled through antique shops in the French Quarter. Locals hunched over in dark, cool neighborhood bars. Even in the Lower Ninth Ward — still scarred by sagging and abandoned houses seven years after Katrina — there was something of a collective ho-hum. “It’s not coming,” said Marie Bryant Farv, a retired nursing school administrator. “It’s going to do the zigzig and, hey, we’ll be okay.”

Farv had just left her 88-year-old mother’s Lower Ninth Ward clapboard house, flanked by an empty lot where a damaged house had been bulldozed and a ramshackle house where a man was squatting despite its deplorable condition. Her mother, Joyce Buckley, had no intention of budging.

Around the corner, Ronnie Brown, 63, a retired construction worker, sat on his sister’s porch, enjoying the afternoon. He fled New Orleans in advance of Katrina, but not this time. “Ain’t goin’ nowhere,” he said. “This one ain’t so bad.”

That kind of nonchalance worries the officials who run this city and surrounding parishes, and they spent much of the day warning about dire possibilities, from trees falling on people who step outside to gauge the wind to pedestrians splashing through deep water only to slip down uncovered manholes and drown. The warnings prompted some to pile bottled water and snack food into shopping carts.

But it was also a day to shrug and laugh at the sky, a mood that was summed up on a sidewalk sign outside the Howlin’ Wolf Den, a pub in the Warehouse District. It read “Isaac Who?”

That scene was a far cry from 2005, when Katrina devastated New Orleans and other parts of the Gulf Coast, leaving nearly 2,000 people dead and exposing the federal government’s abysmal response to the crisis.

Katrina’s storm surge overwhelmed the poorly designed and maintained levee system and left the city flooded and isolated.

The enduring images of the disaster — people clinging to rooftops, desperate masses filling the Superdome and the convention center, the dead floating in the streets — tarnished the Bush presidency.

The tragedy led to a massive reconstruction effort and sparked major changes within FEMA. Fugate, who served as Florida’s top emergency management official when Jeb Bush (R) was governor, assumed control of the agency in 2009 and has taken advantage of congressional changes that expanded the agency’s budget and authorized early deployment of resources and manpower.

These efforts — and Fugate’s stewardship — have earned broad bipartisan praise, most notably from some of Obama’s loudest GOP critics, including Gov. Bobby Jindal (La.) and former Mississippi governor Haley Barbour.

Last week, the agency sent officials to Louisiana to begin coordinating storm response efforts with local officials, and Fugate said Monday that other teams have arrived in Mississippi, Alabama and Florida. In addition, the agency has deployed large supplies of bottled water, food, infant formula and other goods along the storm’s projected path.

Fugate and Knabb also briefed Obama on the situation Monday afternoon. Afterward, Obama convened a call with the governors of Alabama, Louisiana and Mississippi, as well as the mayor of New Orleans, assuring them that they would have any resources they need, according to the White House.

“The biggest lesson we’ve learned is we have to work together as a team at the state and local and federal level,” Fugate said Monday.

But he added that the lessons learned by government officials from Katrina would mean little if residents don’t also prepare for the worst. “Our concern is still individual responsibility and preparedness,” he said. “When those evacuation orders are issued, we need people to heed them and move to higher ground.”

Fugate said that while so much attention, understandably, has focused on New Orleans and the legacy of Katrina, a more immediate danger exists in rural and coastal communities unprotected by the levee system. “I know that Katrina is first and foremost on everybody's mind,” Fugate said. “But I think people need to understand this is not a New Orleans storm; this is a Gulf Coast storm.”

In Tampa, Republican organizers briefly opened their party’s convention on Monday but mostly scrapped the day’s schedule and scrambled to condense the raucous four-day affair into a more subdued three-day event.

It wasn’t the first time that a late-summer hurricane had caused trouble for the GOP gathering. In 2008, presidential nominee John McCain was forced to postpone the start of the party’s convention in Minnesota when Hurricane Gustav threatened to wreak havoc on the Louisiana coast.

Dennis reported from Washington. Ed O’Keefe in Washington contributed to this report.