In its 10 p.m. EDT advisory, the National Hurricane Center in Miami said Isaac's center was about 70 miles west of New Orleans and about 15 south of Baton Rouge, La., with top sustained winds of 60 mph. The storm was moving northwest at 6 mph.
Isaac dumped heavy rain on southeastern Louisiana and southern Mississippi Wednesday and forecasters warned of flooding further inland Wednesday night. A tropical storm warning was posted for Cameron, La., to the Alabama-Florida line.
Isaac was expected to turn toward the north-northwest by Thursday night or early Friday, and the center of the storm was expected to move inland over Louisiana Wednesday and Thursday, before moving over southern Arkansas by early Friday.
Gradual weakening is expected during the next 48 hours.
President Barack Obama Wednesday directed the Federal Emergency Management Agency to make "all available resources" available to support state and local responders, "including any resources to support power restoration efforts once the storm clears," the White House said.
The president directed federal agencies to maintain close contact with state governors' offices.
He held a conference call Wednesday afternoon with Alabama Gov. Robert Bentley, Louisiana Gov. Bobby Jindal, Mississippi Gov. Phil Bryant and New Orleans Mayor Mitch Landrieu, advising the governors of the federal government's preparations and asking them to "identify any additional needs if they arise as the effects of Isaac and the response efforts continue," the White House said.
Speaking at the American Legion national convention in Indianapolis Wednesday, Republican presidential nominee Mitt Romney said the United States "must do all we can" to help people affected by Isaac recover.
"Our thoughts, of course, are with the people of the Gulf Coast states," he said. "Seven years ago today, they were bracing for Hurricane Katrina. This afternoon, they are enduring Isaac. We are grateful that it appears Isaac will spare them from the kind of damage we saw during Katrina. But for many in the Gulf Coast who just finished repairing their homes and are getting life back to normal, this must be a heavy burden."
Isaac has darkened a half million homes and businesses in Louisiana, and the governor of Florida declared an emergency due to flooding from the storm.
Widespread power outages were reported throughout the southern parts of Louisiana, The (New Orleans) Times-Picayune reported.
In St. Bernard Parish, La., more than 16,700 outages had been reported, while more than 10,900 customers were without power in Plaquemines Parish. More than 29,800 homes and businesses were dark in Lafourche Parish.
At 5 a.m., Cleco Power, which serves the north shore, said 15,700 customers had no electricity.
More than 156,000 customers of Entergy New Orleans were without electricity by 7:30 a.m., the utility said on its Web site.
Melonie Hall, director of customer services for the utility, warned in an automated call to customers the storm could take 30 hours to pass and service would be restored "as soon as it is safe to do so."
Extra utility crews had already begun arriving before Isaac slammed ashore late Tuesday.
In Plaquemines Parish, residents with boats have rescued at least six people. Dozens of people were reported stranded by water as high as 9 feet.
The parish's emergency management director said an 18-mile stretch of levee may have been overtopped.
In Florida, water managers were trying to deal with the high water in South Florida left by Isaac Monday, The Palm Beach Post reported.
Gov. Rick Scott attended a briefing in West Palm Beach during which Assistant County Administrator Shannon LaRocque said damage to the county's roads and culverts was estimated at $9.5 million and that number would likely rise.
Wellington City Manager Paul Schofield said there was "no place for the water to go," given the amount of rain left by storms prior to Isaac.
He predicted it would be "many, many days" before things return to normal.
Rain amounts of 7 to 14 inches are predicted over Louisiana, southern Mississippi and southwest Alabama through Friday morning, the National Hurricane Center said. Southern Arkansas could get 3 to 6 inches of precipitation by Friday morning.