NEW ORLEANS (CNN) — Road, rail and air links out of New Orleans began to close Sunday ahead of Hurricane Gustav, which was sweeping toward the Gulf Coast with 115-mph winds Sunday evening.

More than 1.9 million people had fled the city and its surrounding parishes by Sunday night, and fewer than 10,000 people were thought to remain in New Orleans, Louisiana Gov. Bobby Jindal said, citing the city’s police chief. National Guard troops and police were still combing the city broadcasting evacuation messages in English, Vietnamese and Spanish, he said.

Lt. Col. Jerry Sneed, the city’s emergency operations chief, said 18,000 residents without transportation on their own had been evacuated by government agencies — efforts that were stepped up after the catastrophe of Hurricane Katrina in 2005.

But the 24-mile-long causeway over Lake Pontchartrain closed at 7 p.m. (8 p.m. ET) as police topped the bridge — part of the city’s levee system — with sandbags, a police dispatcher told CNN. The last flights out of the city’s airport were slated to take off earlier Sunday evening, and highways out of town had been packed with evacuees from Louisiana and Mississippi all day.

“It was bumper-to-bumper for about 10 hours trying to get out,” said Roberto Ascencio, a restaurant owner in the New Orleans suburb of Gretna. He said he, his wife and daughter packed their pets and whatever else would fit in their car and headed east on Interstate 10 as Gustav neared.

“It was very light when I left my house. I thought it was going to be a piece of cake,” he said. “As soon as we hit the interstate, it was bumper-to-bumper. It was very very slow moving.”

At 8 p.m. ET, forecasters at the National Hurricane Center projected the storm would strike land southwest of New Orleans sometime Monday. The Category 3 storm was about 260 miles (415 km) south-southeast of the city, the National Hurricane Center in Miami reported, and it was moving across the central Gulf of Mexico at 17 mph.

The storm was packing top winds of 115 mph (185 km/h), with hurricane-force winds extending about 70 miles from the eye.

Meanwhile, three years after his response to Hurricane Katrina received widespread criticism, President Bush canceled plans to attend this week’s Republican National Convention as his administration scrambled to prepare for Gustav’s arrival. The GOP also curtailed its plans for the convention as the storm neared.

In Mississippi, which took the brunt of Katrina’s winds and storm surge three years ago, Gov. Haley Barbour said government agencies were “10 times better prepared” than before — but “that doesn’t mean everything is going to go right,” he said.

“Anybody who thinks everything is going to go perfect just doesn’t know what they’re talking about,” Barbour said.

Maps of Gustav’s path show that it could strike southern Louisiana and other areas battered by Hurricanes Katrina and Rita in 2005. Katrina, a Category 3 storm, flooded most of New Orleans, flattened beach towns in Mississippi and killed more than 1,800 people.

Jindal said the city’s protective levees “will barely hold or barely be overtopped” if forecasters’ predictions hold. But the weaknesses remaining in Louisiana’s levee system since Katrina could leave Louisiana open to an even worse disaster unless Gustav changes course, a leading researcher said Sunday.

Louisiana State University Professor Ivor van Heerden said state and federal officials could have done a lot more to repair damaged levees and restore the coastal wetlands and barrier islands that can blunt the impact of a hurricane in the intervening three years.

“If the models are correct, Gustav will destroy what Katrina and Rita did not,” van Heerden said. “This is going to be flooding of a much larger area than Katrina.”

As deputy director of LSU’s Hurricane Center, van Heerden warned of the catastrophic consequences a major hurricane would have on New Orleans long before Katrina struck three years ago.

Gustav already has been blamed for more than 60 deaths in the Caribbean, including 51 in southwestern Haiti. It struck Haiti, the Dominican Republic, Jamaica and Cuba before crossing into the Gulf of Mexico and growing into a major hurricane.

New Orleans Mayor Ray Nagin ordered a mandatory evacuation and a curfew and warned against looting. Nagin, who was widely criticized for failing to prepare for Katrina, said he curfew would continue until the threat of the storm passes, and he warned that looters would be dealt with harshly.

“Anybody who’s caught looting in the city of New Orleans will go directly to Angola,” he said, referring to Louisiana’s notorious maximum-security prison.

But some, like Louisiana resident Nick Dominque, planned to remain behind. Dominque, 30, told CNN that he had weathered Hurricanes Rita and Katrina and needed to look after his parents in the southern part of the state.

“They think that they can handle any storm that hits, so I’m going to bunker down and stay with them,” he said. “I never thought after Rita that I would try to ride out another storm. But here I am again.”

Homeland Security Secretary Michael Chertoff, who was overseeing evacuation efforts from New Orleans, said most people have chosen to leave, either by car, bus, ambulance or air.

“I was reading in the paper about people who say, ‘No I’m going to stay.’ I think that’s pretty stupid, but I can’t stop that from happening,” he said. “I’m hoping it’s a very small number of people.”

Bush warned Gulf Coast residents Sunday that a “serious” storm was headed their way and announced plans to head to Texas to meet with emergency workers and evacuees. Both the president and Vice President Dick Cheney canceled plans to attend the Republican convention in St. Paul, Minnesota, and convention events have been cut back sharply as the storm approached.

“This is a time when we have to do away with our party politics and we have to act as Americans,” said Arizona Sen. John McCain, the party’s presidential nominee-in-waiting. The party would suspend most of Monday’s activities “except for those absolutely necessary,” McCain said from St. Louis, Missouri.

Republican National Committee Chairman Mike Duncan said the convention would have to be called to order and its credentials committee report received so delegates can formally nominate McCain and his running mate, Alaska Gov. Sarah Palin.

“I would say that the main information I can give you today is that owing to the fact that the senator has asked us to take our Republican hats off and put our American hats on, tomorrow’s program will be business only, and we’ll refrain from any political rhetoric that would be traditional in an opening session of a convention,” McCain campaign adviser Rick Davis said.