"March Madness helped deliver a huge month for Illinois, but March is essentially a 'last hurrah' for the state's rapid growth," said Joe Boozell, analyst for PlayIllinois.com. "Illinois will still be one of the largest U.S. markets because of the population of the state, but it will be difficult to maintain its current status as the U.S. No. 3, much less catch Nevada. No matter how appealing a market, there isn't any easy way to overcome the inconvenience of in-person registration."

Betting at Illinois's retail and online sportsbooks surged to $633.6 million in March, according to official data released Monday. That is up 24.3% from $509.8 million in February and easily topped the previous record $581.6 million in January. Bettors placed $20.4 million per day in the 31 days of March.

March's high volume produced a record $49.9 million in operator revenue, topping $49.4 million in January. The month's win created $44.3 million in taxable revenue, which yielded $6.6 million in state taxes and $493,543 in local taxes.

Illinois surpassed Pennsylvania ($560.3 million) in monthly handle for the second straight month and gained ground on New Jersey ($859.6 million) and Nevada ($641 million). In doing so, the state surpassed $3 billion ($3.6 billion) in lifetime handle.

"The growth over the last year has been staggering, making Illinois the quickest to $3 billion in U.S. history," said Jessica Welman, analyst for PlayIllinois. "The question now becomes what will happen when the brakes are hit on that growth? Does it alarm lawmakers enough to make a change? Or will Illinois be satisfied with where the sports betting market has been frozen in place?"

The NCAA Tournament was the main driver in March, generating an estimated $176.8 million in bets even as bettors were barred from wagering on Illinois and Loyola Chicago. The NBA also drew well, helping to push basketball betting to $365.7 million. The previous high for basketball in a month was $256.7 million.

"Illinois has grown in some unique ways, but the state's love of basketball has really carried the state over the last few months," Boozell said. "If the ban on betting on in-state college teams didn't exist, Illinois would have likely been the No. 2 market in the U.S. in March."

Online wagering accounted for 96.1%, or $609 million, of all bets in March.

DraftKings/Casino Queen represented 32.2% of the state's entire handle in March, generating $207.6 million in online and retail wagers. $203.9 million of March's bets came online.

FanDuel/Fairmont attracted $195.2 million in bets — $194.9 million of which came online. BetRivers/Rivers Casino continued to lose pace with the market leaders, generating $95.7 million in online betting and $106.9 million overall.

"One of the biggest issues with the return of in-person registration is it puts newly launched operators at a permanent disadvantage," Welman said. "In one way, Barstool was fortunate to launch when it did, helping them build a foundation. But growth will be difficult for all operators, and especially those that are farther away from the largest population centers."