Presley backstage at the International Hotel on Aug. 1, 1969.

The first time Elvis Presley played Las Vegas, the critics hated him. Appearing in the Venus Room at the New Frontier Hotel in the spring of 1956, the 21-year-old hillbilly crooner with the dreamy eyes and well-lubricated hips was billed as "America's only atomic-powered singer." Two years earlier, he had been driving a truck in Memphis for $45 a week. Now he had a contract with RCA Victor Recordings and a movie deal with Paramount Studios.

At the time, Vegas was known for its mobbed-up glamour -- bent-nose guys in fedoras, ladies in white gloves. It was the era of The Rat Pack. The furniture was mid-century modern, the patrons were well-dressed, the food and drink were expensive, and the nightclubs were ruled by hepcats like Frank Sinatra and Dean Martin.

Elvis Presley's Billboard Chart Records

The audiences packing the venues, however, were somewhat old by today's standards, mostly in their 50s and 60s. After World War II, veterans had gotten a late start on life. The youth culture pioneered by their children, the baby boomers, was still in its embryonic stage. And still misunderstood.

"As he stands up there clutching his guitar, he shakes and shivers like he is suffering from itchy underwear and hot shoes," wrote Ralph Pearl in the Las Vegas Sun.

"For the average Vegas spender or showgoer, [Elvis is] a bore," wrote another of the Sun's critics, Bill Willard.

Betting On Vegas: The Billboard Photo Shoot featuring Wayne Newton, David Guetta & More

Today, a trip to Vegas confirms Elvis' enduring influence. Elvis impersonators perform at the Flamingo Hotel, Harrah's Hotel and Planet Hollywood. Costumed Elvises distribute broadsheets for escort services. There's an Elvis Suite at the Las Vegas Hotel, and an annual Elvis festival and parade -- former Mayor Oscar Goodman and his wife, the current mayor, Carolyn Goodman, traditionally ride in a pink Cadillac with an Elvis impersonator. At the Hard Rock Cafe are gold records, belts, a smashed guitar and a telegram from Elvis and his manager, Colonel Tom Parker, to The Beatles.

Elvis performing in 1975 at age 40.

"Elvis signaled an important change in Las Vegas," says Oscar Goodman. During his 12-year administration, the city became known for its risque slogan: "What happens in Vegas stays in Vegas." "Elvis bridged the gap between the Frank Sinatra/Rat Pack years and what you see today -- big names, big shows, big packed houses. More than any other performer, Elvis set the tone."

Inspired in part by Elvis' quickie marriage to Ann-Margret in the 1964 schmaltz classic Viva Las Vegas, there are at least a half dozen chapels offering Elvis weddings. (The title song from the movie is the city's unofficial theme.) At Planet Hollywood, an Elvis will officiate and sing up to three songs. (The property that Planet Hollywood occupies today was once the Aladdin Hotel, where Presley, then 32, married the 21-year-old Priscilla Ann Beaulieu, in 1967, the first marriage for both.)

Recently, the Westgate Las Vegas Resort -- formerly the International Hotel, where Elvis posted a record 837 consecutive ­sold-out performances over seven years, in front of 2.5 million total people -- announced plans to cooperate with Elvis' Graceland estate to bring a permanent exhibit to the hotel.

Rewinding the Charts: 45 Years Ago, Elvis Presley Ruled the Hot 100 for the Last Time

"Basically, Elvis never died," says Travis Allen, a former electrician who appears nightly at Planet Hollywood as Elvis. Allen points out that he is the only solo Elvis with a live band playing a major hotel venue. His material spans all of the Elvis periods, from young and handsome to the bloated version in the spangled jumpsuit.

"People come up to me every night and say, 'Thank you for keeping the memory alive,' " says Allen. "No other entertainer ever had his hold on an audience."

On July 31, 1969 -- a year after his big comeback special on TV re-energized his career -- Elvis performed his first sold-out show at the International.

This time the reviews were breathless:

"Elvis got a constant, roaring approval from his fans who all but threw themselves into the aisles and out of the balcony as the Pelvis sang his many rock'n'roll hits while fiercely, almost savagely turning himself outside in," wrote Pearl at the Sun.

"His aura was incredible. It made you kind of goose-pimply," says Carolyn Goodman, who attended his shows and met The King several times.

Elvis with Sammy Davis Jr. (left) backstage at the Showroom at the International in 1970.

Elvis Presley's 1974 Memphis Concert Remembered With 2 CD Set

The next day, Parker negotiated a five-year contract for Elvis to play each February and August, at an annual salary of $1 million.

During the next seven years, Elvis ruled the Strip. Tickets to his dinner show were $17.50 and included lobster or steak. The midnight show included drinks and cost a bit less. In one 29-day period, Elvis played for 101,509 guests. Over seven years, he is said to have sold $43.7 million in tickets, an estimated $171 million in 2015 dollars.

As the years passed, and Elvis morphed -- from hunky to husky to grossly obese -- so too evolved the culture of Vegas and the nation, though not entirely in the same direction. In a sense, Elvis represented an innocent era, when the nation's youth were first beginning to separate from the culture of their elders -- what would come to be known as the generation gap. When the war in Vietnam upped the ante, with the mandatory draft and 58,220 dead, Elvis' innocent love ballads and catchy dance tunes would give way to the anger and passion of the Woodstock generation's rockers, who took their form from R&B but added new and more powerful content.

But Vegas was not Altamont or even the Fillmore; the city of sin that the mob had built in the desert was isolated, well-policed and pricey. Although Elvis brought in younger audiences with money to spend, he didn't bring hippies or rock bands. More than anything, what he did was make it cool for fading superstar performers to find a second (or even third) act of their career in Vegas. In that way, Elvis paved the way for the likes of Britney Spears.

The King's well-documented taste for the good life also helped set another kind of tone in Vegas: as an oasis of over­indulgence. Dean Martin and the Rat Packers were playful drunks; sweaty Elvis set a whole new standard of debauchery.

Toward the end of his career, Elvis was said to be addicted to a variety of prescription drugs from five different doctors. His frequent crash diets also took a toll, as did his endless schedule, the death of his beloved mother and the breakup of his marriage to Priscilla in 1972. Oscar Goodman came to know Elvis during visits to the town's leading physician, Dr. Elias Ghanem ("I had a severe case of gout and Elvis was always being treated for various maladies").

Elvis Presley Graceland Exhibit Heading to Las Vegas

According to Jody Ghanem, the ­doctor's widow, Elvis stayed for weeks at a time at the Ghanem home, as the doctor attempted to wean him from his medications. Elvis gifted the doctor a 1971 Stutz Bearcat and several rings, one a big horseshoe with diamonds. "One time, Elias took Elvis' medicines and emptied them out and replaced them with sugar capsules," says Ghanem, a former Radio City Rockette. "Elvis was superstitious. He had this premonition that he was going to pass away in his 40s, the same age as his mom."

Elvis' last performances in Vegas were Dec. 2 through 12 in 1976. After returning to Graceland in Memphis, he died on Aug. 16, 1977, at the age of 42.

"Elvis might be gone, but he ain't never left the building," says Elvis impersonator Allen, putting on his best Memphis drawl.

This story originally appeared in the March 21st issue of Billboard.