Not long ago, bank executives would compete with one another to be the loudest critic of cryptocurrencies.

Jamie Dimon, the chief executive of JPMorgan Chase, once compared Bitcoin to a pet rock and said the whole crypto industry should be banned. Bank of America’s Brian Moynihan described the space as an “untraceable tool for money laundering,” while HSBC’s chief executive proclaimed bluntly: “We are not into Bitcoin.”

Now big banks can’t stop talking about crypto.

In investor calls, public presentations and meetings with Washington regulators, financial executives are tripping over one another to unveil new plans — including the development of fresh cryptocurrencies under bank umbrellas and loans tied to digital assets.

There’s no small mix of political opportunism at play, given that President Trump and his family are vociferous crypto boosters and investors. And of course there is a degree of old-fashioned jealousy among the traditional finance set at the riches earned by onetime fringe companies and investors as Bitcoin more than doubled over the past year to blow past $100,000.

But behind the scenes at major financial institutions — and in stark contrast to the public showboating among chief executives — fear is also rising that the rush into crypto may risk the safety of personal bank accounts in ways that Wall Street and Washington are just beginning to understand.

The worries, described by nine Wall Street executives briefed on their organizations’ crypto initiatives but not authorized to speak publicly for their employers, center on the creation of a new interbank checking account and payments system built on crypto and blockchain technology. That system would come with few consumer protections and nascent regulatory oversight.

The system, being sketched out by top executives and lawyers at huge banks, including JPMorgan, Bank of America and Citi, involves a complicated corner of the crypto ecosystem called a stablecoin.

Stablecoins could upend the old financial order.

Stablecoins work like a digital i.o.u. Their value is pegged to the U.S. dollar, unlike cryptocurrencies like Bitcoin, which have no such constraint and thus can swing wildly in price.

Here’s how they would work at scale: A bank customer places his or her cash with the bank and gets a stash of stablecoins in return. The consumer can then use these coins, for instance, to send money overseas or make international payments less expensively than wiring money.

The funds that a customer exchanges in return for stablecoins is, to the bank, the equivalent of a guaranteed profit.

That’s because a federal law passed this summer with bipartisan support requires banks to take the money they receive for stablecoins and invest it in government bonds and other virtually risk-free assets. Those bonds generate interest, which the bank keeps. Unlike traditional bank accounts, these savings don’t earn even nominal interest for depositors.

Another big change: Stablecoins eschew the century-old practice of automatic federal deposit insurance. If they fail, there is no guarantee of a government backstop.

Bankers say stablecoins, if widely adopted, could bring a radical change to the nuts and bolts of their industry, and they have the potential to upend a century of accepted banking practices.

One reason is that the money that a customer places with a bank in exchange for a stablecoin cannot be lent out in the same way that money placed in a traditional checking and savings account can be.

Any dollar that goes into a stablecoin and not a consumer’s traditional bank account essentially shrinks the size of a bank’s lending book and the bank’s deposit base overall. This means banks could have fewer deposits to make home or business loans with, which the Federal Reserve Bank of Kansas City last week warned could carry unintended consequences for the economy.

“The genie is out of the bottle,” said Mike Cagney, a former chief executive of SoFi and now the head of the digital lender Figure. He predicted that the rise of stablecoins would come at the expense of bank deposits. “You don’t need a lot of deposit flight to really buckle the banks,” Mr. Cagney said.

Not everyone agrees that the result will be cataclysmic.

“The consumer checking account is probably safe,” said Tim Spence, the chief executive of Fifth Third, a Cincinnati bank with $210 billion in assets that traces its roots back to 1858. It plans to accept stablecoins issued by a large group of banks.

Privately, however, many lending veterans are vexed at how rapidly changes are coming.

“If the banking industry was being totally frank,” one prominent banking attorney said, “they would say they wish stablecoins had never been invented.”

Wildcat banks offer a warning.

Wall Street has been nibbling around the edges of crypto for years.

Investment banks have already been advising crypto companies with public listings and bond offerings (collecting fees in the process). Goldman Sachs has offered wealthy clients loans based on their crypto holdings since 2022.

The big banks’ brokerage units help major clients buy investment funds with exposure to cryptocurrency, and in some instances they help the clients directly hold crypto coins themselves.

The first Trump administration and Biden years were a step back, however, as banks cut off crypto companies and investors from their networks amid pressure from skeptical regulators.

Eventually JPMorgan, despite Mr. Dimon’s public crypto antipathy, built a team of several hundred employees to study how to make money from crypto and began testing a niche digital asset, JPM Coin, to experiment with digital payments for commercial clients. It didn’t take off.

JPMorgan in recent months has assigned a team of internal researchers to unearth legal documents related to the era of so-called wildcat banking from two centuries ago. In that era, hundreds of state-chartered banks issued their own currencies. JPMorgan’s researchers were looking into whether some of those dormant laws applied to the bank’s plan to create its own stablecoin today.

They uncovered something else, though. In the wildcat era, fraud and bank failures ruled. That tumult was fixed only by the creation of the first U.S. National Bank note, a predecessor to the modern U.S. dollar. It was issued only by official national chartered banks — one of which was the precursor to JPMorgan Chase.

The GENIUS Act codified an untested future.

Mr. Trump, who, like the nation’s biggest bankers, used to be a crypto skeptic, has worked during his second term to bring crypto into the mainstream.

This summer, Mr. Trump signed into law the GENIUS Act, passed by Congress during what Republicans called “cryptocurrency week.”

The law created a pathway for banks to operate in stablecoins tied to the U.S. dollar, long considered the steadiest global currency because of the reliability of the American economy.

It requires stablecoin issuers to maintain “reserves” of U.S. Treasury bonds or dollars equal to the total value of the coins they have distributed.

The GENIUS Act enjoyed a measure of bipartisan backing. Senator Kirsten Gillibrand, Democrat of New York, said it unlocked “the next generation of financial innovation.”

Critics of the law pointed out that Mr. Trump’s sons run a crypto start-up, World Liberty Financial, that issues its own stablecoin and stands to benefit handsomely if the digital currency takes off.

Bank lobbyists didn’t just support the legislation, but took an active role in shaping it.

Bankers who participated in the lobbying said they felt somewhat forced by circumstance. Stablecoins, they said, were coming fast — both from new upstarts like Circle and big retailers like Walmart and Amazon, which are said to be studying creating coins of their own.

If such competitors become popular, they will act as a runaround of banks large and small, by allowing consumers to buy goods and pay for services without needing an account at an existing lender.

Thus banks have been discussing with one another how to create one big stablecoin of their own. (The name, the bankers briefed on it said, is to be determined.) They are also hedging their bets, developing individual ones if needed. Some plan to operate rewards programs, similar to existing credit card promotions, to woo customers to their own coins.

None of the coins are expected to be in use before the end of the year.

Mr. Moynihan of Bank of America confirmed during a quarterly earnings call last month that his lender was brushing up on stablecoins, saying that “both the industry and ourselves will have responses.”

He said he had no clue how popular they would become, or whether the bank would go it alone or in concert with other lenders.

“It will be a complex array,” Mr. Moynihan said.

“Hopefully not complex to the consumer, frankly,” he added.

Audio produced by Patricia Sulbarán.