When Honolulu’s Alston Hunt Floyd & Ing merged with the giant international law firm Dentons in May, it seemed the Hawaii firm was going global.

Billed as the world’s largest law firm, Dentons has nearly 900 lawyers in the U.S. and thousands more in offices around the world, including 3,500 lawyers in China alone.

But now a rival Honolulu lawyer is challenging the new AHFI-Dentons partnership’s ability to practice law in Hawaii. The question is whether a Hawaii Supreme Court rule governing law firms prevents a multi-state entity from practicing in Hawaii if any of the firm’s partners aren’t licensed here.

“The answer to the question is clear: Dentons is not ‘authorized’ to practice law in Hawaii,” Honolulu attorney Jim Bickerton argues in his brief before the Hawaii Supreme Court.

Although the AHFI-Dentons merger may bring a uniquely giant firm to Honolulu, there’s nothing unique about a Honolulu firm merging with a mainland counterpart, said Paul Alston, AHFI’s founder. He chalked up Bickerton’s challenge to rivalry.

“He and we are opponents in probably 50 cases, maybe more,” Alston said. “He constantly is mad at me because when we show up, we don’t fall down and have our client hand him a check.”

Bickerton has his own choice words for Dentons, calling one of the firm’s rebuttals over the current dispute a “dumb-ass argument.”

One thing that’s not in dispute is that lawyers are highly regulated.

In Hawaii, there’s a statute generally limiting who can practice law here, plus general Rules of Professional Conduct. The rule Bickerton cites is part of the Rules of the Supreme Court of Hawaii, which encompass topics like admission to the Hawaii bar, disciplinary rules for lawyers and rules governing the Hawaii State Bar Association.

These rules also say who can own law firms doing business in Hawaii. Shares in a Hawaii firm, the rules say, “may be owned only by a lawyers’ professional business organization or by one or more persons licensed to practice law in this state by this court.”

Bickerton argues this means that only lawyers licensed to practice in Hawaii can own stock in a Hawaii firm. Accordingly, he argues, Dentons can’t practice in Hawaii and neither can former AHFI lawyers, who are now part of Dentons.

Dentons has fired back with an 88-page brief filed last week. Among other arguments, Dentons points out that other Honolulu law firms, including Carlsmith Ball, have merged with out-of-state firms and thus have non-Hawaii lawyers as partners.

“They’re illegal then,” Bickerton said of Carlsmith Ball, which is Hawaii’s oldest law firm. “Carlsmith should be shut down too.”

Laura Lucas, Carlsmith Ball’s managing partner, did not return calls and an email seeking comment.

In its brief, Dentons points to a 2015 case brought by the Hawaii Office of Disciplinary Counsel exploring the same issue. In that case, a disciplinary board hearing officer dismissed the ODC’s complaint, determining that the Supreme Court’s rule violated the U.S. Constitution’s Commerce Clause by essentially infringing on interstate commerce between lawyers.

Although the disciplinary counsel could have appealed the hearing officer’s ruling to court, the counsel chose not to do so, which means Hawaii courts have yet to weigh in. As a result, Bickerton notes the hearing officer’s decision isn’t binding on the courts; Alston acknowledges the point, but says the hearing officer’s order is persuasive.

In any case, Dentons makes similar arguments concerning the Commerce Clause, among others, in its brief.

The Hawaii Supreme Court justices face a thorny question that will determine the future of multi-state firms in Hawaii. Bickerton said he just wants the court to take a hard look at its rule, even if that means the court concluding the rule violates the Commerce Clause.

“Let the Supreme Court declare that their own rule is unconstitutional,” he said.

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