Over the last decade, for example, New Hampshire has passed nearly a dozen laws affecting trusts that expanded their life span, lowered taxes and made it easier to transfer assets. In 2013, the state created a special trust court subdivision to handle the complex litigation; last year, an overhaul of state banking laws simplified regulations.

Still, Nevada “is definitely the most aggressive in my experience in terms of asset protection,” Mr. McCaffrey said.

Starting with the absence of any state income tax and resilient secrecy protections, Nevada has added a passel of laws and regulations intended to lure trust business. Individuals who establish so-called irrevocable trusts have more flexibility to transfer assets to a new trust with more favorable terms. Creditors are blocked from access to money held in trusts (making the arrangement also popular among doctors, who worry a malpractice case could bankrupt them). And what are known as dynasty trusts allow the wealthy to pass their fortunes from generation to generation for hundreds of years without paying estate taxes.

Defenders of the industry won what was characterized as a “do or die” battle in 2013, fending off legislation that would have opened up Nevada trusts to future claims from spouses and domestic partners for property settlements, alimony or child support. And Nevada has only a narrow two-year window for creditors to make a claim against a trust’s assets.

These sorts of loopholes have been criticized by some legal scholars as “not morally defensible,” but defenders dismissed the potential for abuse.

“It’s against the law in this state to conceal assets from legitimate creditors,” said Mark A. Hutchison, Nevada’s Republican lieutenant governor, a lawyer himself.

“We’ve got to be aware of phantom threats that could blow up our industry,” he added.

Phantom threats, he said, include recent suspicions — after the leak of the Panama Papers — that citizens and foreigners are illegally hiding assets in the state.