NEW ORLEANS (AP) — With a historic evacuation of nearly 2 million people from the Louisiana coast complete, gun-toting police and National Guardsmen stood watch as rain started to fall on this city’s empty streets Sunday night — and even presidential politics took a back seat as the nation waited to see if Hurricane Gustav would be another Katrina.

The storm was set to crash ashore late Monday morning with frightful force, testing the three years of planning and rebuilding that followed Katrina’s devastating blow to the Gulf Coast. The storm has already killed at least 94 people on its path through the Caribbean.

Painfully aware of the failings that led to more than 1,600 deaths during Katrina, this time officials moved beyond merely insisting tourists and residents leave south Louisiana. They threatened arrest, loaded thousands onto buses and warned that anyone who remained behind would not be rescued.

‘‘Looters will go directly to jail. You will not get a pass this time,’’ Mayor Ray Nagin said. ‘‘You will not have a temporary stay in the city. You will go directly to the Big House.’’

Col. Mike Edmondson, state police commander, said he believed that 90 percent of the population had fled the Louisiana coast. The exodus of 1.9 million people is the largest evacuation in state history, and thousands more had left from Mississippi, Alabama and flood-prone southeast Texas.

Late Sunday, Gov. Bobby Jindal issued one last plea to the roughly 100,000 people still left on the coast: ‘‘If you’ve not evacuated, please do so. There are still a few hours left.’’

Louisiana and Mississippi temporarily changed traffic flow so all highway lanes led away from the coast, and cars were packed bumper-to-bumper. Stores and restaurants shut down, hotels closed and windows were boarded up. Some who planned to stay changed their mind at the last second, not willing to risk the worst.

‘‘I was trying to get situated at home. I was trying to get things so it would be halfway safe,’’ said 46-year-old painter Jerry Williams, who showed up at the city’s Union Station to catch one of the last buses out of town. ‘‘You’re torn. Do you leave it and worry about it, or do you stay and worry about living?’’

There were frightening comparisons between Gustav and Katrina, which flooded 80 percent of New Orleans. There was no doubt the storm posed a major threat to the partially rebuilt city and the flood-prone coasts of Louisiana and southeast Texas.

Mindful of the potential for disaster, the Republican Party scaled back its normally jubilant convention — set to kick off as Gustav crashed ashore. President Bush said he would skip the convention altogether, and Sen. John McCain visited Jackson, Miss., on Sunday as his campaign rewrote the script for the convention to emphasize a commitment to helping people.

The nation’s economic attention was focused on Gustav’s effect on refineries and offshore petroleum production rigs. The combination of prolonged production interruptions, such as occurred when Katrina and Rita damaged the Gulf infrastructure, could trigger rising prices.

Billions of dollars were at stake in other wide-ranging economic sectors, including sugar harvesting, the shipping business and tourism. The Mississippi Gaming Commission ordered a dozen casinos to close.

Forecasters said Gustav could strengthen slightly as it marched toward the coast. At 11 p.m. EDT Sunday, the National Hurricane Center said Gustav was centered about 220 miles southeast of New Orleans and was moving northwest near 16 mph. It had top sustained winds of 115 mph, and was likely to stay a Category 3 storm when it made landfall west of New Orleans. Category 3 storms have winds between 111 mph and 130 mph.

Rain started falling in New Orleans before sunset, and tropical storm-force winds had reached the southeastern tip of the state.

New Orleans will likely be on the ‘‘dirty’’ side of the storm — where rainfall is heaviest and tornadoes are possible, but the storm surge is lower. If forecasts hold, the city would experience a storm surge of only 4 to 6 feet, compared to a surge of 10 to 14 feet at the site of landfall, said Corey Walton, a hurricane support meteorologist with the National Hurricane Center. Katrina, by comparison, brought a storm surge of 25 feet.

Surge models suggest large areas of southeast Louisiana, including parts of the greater New Orleans area, could be flooded by several feet of water. But Gustav appears most likely to overwhelm the levees west of the city that have for decades been underfunded and neglected and are years from an update.

Against all warnings, some gambled and decided to face the storm’s wrath. On an otherwise deserted commercial block of downtown Lafayette, about 135 miles west of the city, Tim Schooler removed the awnings from his photography studio. He thought about evacuating Sunday before deciding he was better off riding out the storm at home with his wife, Nona.

‘‘There’s really no place to go. All the hotels are booked up to Little Rock and beyond,’’ he said. ‘‘We’re just hoping for the best.’’

The final train out of New Orleans left with fewer than 100 people on board, while one of the last buses to make the rounds of the city pulled into Union Station empty. Police made final rounds around 7 p.m. Every officer in the department was on duty, and the 1,200 on the street were joined by 1,500 National Guardsmen.

The only sign of life on St. Bernard Avenue — a four-lane artery through the partially rebuilt Gentilly neighborhood that flooded during Katrina — was a brown and black rooster meandering along the street.

‘‘When the 911 calls start coming in, we’ll know how many people are left in town,’’ said police superintendent Warren Riley.

Even as they pressed to complete the evacuation, officials insisted there would be no repeat of the inept response to Katrina’s wrath. Homeland Security Secretary Michael Chertoff said search and rescue will be the top priority once Gustav passes — high-water vehicles, helicopters and fixed-wing aircraft, Coast Guard cutters and a Navy vessel that is essentially a floating emergency room are posted around the strike zone.

West of New Orleans in Houma, he wished passengers well as stragglers boarded buses for Shreveport and Dallas.

‘‘It’s going to be hot on some of the buses. It’s going to be a long trip,’’ Chertoff said. ‘‘So it’s not going to be pleasant, but it’s a lot better than sitting in the Superdome and it’s a lot better than sitting in your house.’’

Melissa Lee, who lives in Pearl River, a town near the boundary of Mississippi and Louisiana, was driving away to Florida Sunday. Before she left, she heard neighbors chopping down trees with chain saws, trying to ensure the tall pines that surrounded their homes wouldn’t come crashing down.

‘‘I sent my son out with a camera and said, ‘Go take pictures of our backyard. Because it’s going to look different when we get back.’’’

———

Associated Press writers Janet McConnaughey, Robert Tanner, Cain Burdeau, Alan Sayre, Allen G. Breed and Mary Foster contributed to this report from New Orleans. Vicki Smith in Houma, Doug Simpson in Baton Rouge and Michael Kunzelman in Lafayette also contributed. Kelli Kennedy reported from Miami, and Shelia Byrd contributed from Pearl, Miss.