With the U.S. Supreme Court expected to hear arguments soon on a case that could upend the country's sports gambling laws, for the first time most Americans support making wagering on professional sports legal, according to new poll conducted by The Washington Post and the University of Massachusetts Lowell.

A 55-percent majority approve of legalizing betting on pro sporting events, a flip from almost a quarter century ago, when a federal law went into effect banning the practice in most of the country and 56 percent of Americans disapproved of legalization in a Gallup/CNN/USA Today poll.

[Read full poll results | How the poll was conducted]

While the issue has been debated for decades, the coming months figure to be pivotal for sports gambling in the United States. With the Supreme Court agreeing to hear arguments on the state of New Jersey’s efforts to legalize sports wagering, major stakeholders from the professional sports world increasingly have showed an openness to the idea, and some see a growing sense of inevitably surrounding an issue that was contentious and divisive not long ago.

“Literally, we’re at the 1-yard line, and it’s first-and-goal,” said Daniel Wallach, a sports gaming law expert and attorney at Becker & Poliakoff in Fort Lauderdale. “That’s how close it is.”

The American Gaming Association is scheduled to hold a briefing on Capitol Hill on Wednesday, and public sentiment appears to have moved in a direction that could make it easier for the courts or lawmakers — or both — to reconsider federal legislation that largely limits sports betting to Las Vegas. According to the new poll, the increase in support is broad-based and cuts across most demographics, with support among men and women, young and old and those from lower- and higher-income households.

According to the Post-UMass Lowell poll, just more than 1 in 5 sports fans (21 percent) have bet on professional sporting events in the past five years, and they’re more likely to be avid sports fans, men, pro football fans, nonwhites, and under 40 years old.

Support for legalization is highest among the those who’ve placed a sports bet in the past five years (84 percent), and is nearly as high among fans who have played in a fantasy sports league (79 percent), avid sports fans (70 percent), men (63 percent), people with household incomes of $100,000 or more (61 percent) and pro football fans (60 percent).There’s little partisan difference on the issue, with 52 percent of Republicans and 57 percent of Democrats in support of legalizing sports gambling.

Opposition to sports gambling appears to be fading with older generations. In 1993, 29 percent of Americans ages 50 and older supported legalizing sports gambling, as did 48 percent of adults below that age. Today, 46 percent of those over age 50 support legalization, rising to 62 percent of people ages 18 to 49.

The evolution in public sentiment reflects a similar shift from many of the major decision-makers throughout the sports world, an industry in which gambling was once considered a cardinal sin. Professional leagues have either pivoted on the issue or at least made moves to acknowledge change could be afoot. The NHL will open its next season with a franchise based in Las Vegas, and the NFL’s Oakland Raiders will be relocating there as early as 2019. NBA Commissioner Adam Silver has openly discussed legalizing gambling, as has Major League Baseball, which has partnered with Genius Sports, an integrity company that monitors baseball wagering.

“We’re in the process of talking to our owners and figuring out where we want to be in the event that there is in fact a significant change coming,” MLB Commissioner Rob Manfred said in July.

The evolving fan sentiment signifies a remarkable shift. Historic Gallup polling from 1951 found 38 percent favored national legalization of gambling on “horse races, lotteries and numbers,” and gambling overall has gained public support in the decades since then. A 1994 Public Perspective examination of public opinion found that while gambling grew in support, support for sports betting did not. By the end of the 1980s, in fact, fewer accepted sports gambling than they did at the beginning of the decade — coinciding with Pete Rose’s lifetime ban from baseball in August 1989 after it was found he gambled on the sport.

In 1992, Congress passed the Professional and Amateur Sports Protection Act (PASPA), which banned sports betting in all but a handful of places. But gambling proponents say hundreds of billions of dollars are wagered on sports illegally each year in an unregulated market, and in the 25 years since PASPA was signed into law, outlets for gambling have become only more accessible.

“As our industry expanded, we confronted a lot of fears, a lot of concerns, and what time has shown is that those fears and concerns were often misplaced,” said Geoff Freeman, the president and CEO of the American Gaming Association.

Recent polling has tracked a rise in support for legalizing sports gambling. Fairleigh Dickinson University polls found a slim 53 percent majority of Americans in 2010 opposed to sports betting in all states with 39 percent in favor of it. But by 2016, more supported national legalization than opposed it, 48 percent to 39 percent, a trend that appears to continue in the new Post-UMass Lowell poll, the highest level of support tracked by the Roper Center public opinion archive. The survey was conducted August 14-21, among a random national sample of 1,000 adults reached on cell phone and landline phones. The margin of sampling error for overall results is plus or minus 3.7 percentage points.

Freeman points to three key reasons for the marked changes in American attitudes related to sports gambling in the past quarter-century: the rise of the Internet, the prevalence of casinos across the country and the popularity of fantasy sports.

While casinos might have normalized gambling for many and the Internet has made access to betting lines and bookies easier than ever, fantasy sports particularly has struck a chord with both gamblers and sports fans, allowing them to win money based on the performances of individual players.

According to the Post-UMass Lowell poll, 16 percent of sports fans have participated in fantasy sports leagues in the past five years, and playing in fantasy leagues is more popular among avid sports fans, men, people under 40 and college graduates. Perhaps most telling: There’s significant overlap between sports bettors and fantasy players — 56 percent of fantasy players have made bets on professional sports in the past five years, while 42 percent of sports bettors have played fantasy sports in that same time period.

“It begs a lot of questions for people,” Freeman said of fantasy sports. “I heard it from owners in sports, I heard it from fans, I heard it across the board: What’s the difference between this and betting on sports? The simple answer is, there isn’t a big one.”

Scott Clement contributed to this report.