Wall Street had heard it before: Donald Trump is considering whether to revive the Depression-era Glass-Steagall Act that separated investment and commercial banks.

That may be why stocks in U.S. banks were largely unfazed by Trump's statements in a Bloomberg News interview Monday that 'there's some people that want to go back to the old system" and "we're going to look at that."

The KBW Bank Index, which tracks 24 financial institutions, climbed 0.9% at the close of trading, while global firms from JPMorgan Chase (JPM - Get Report) to Goldman Sachs (GS - Get Report) and Bank of America (BAC - Get Report) all posted slight gains. Traders familiar with the president's previously articulated position speculated both about how serious he was and whether any revision of the law would go as far as the previous one.

The move would prove particularly costly to individual customers, who would lose out on advances such as mobile banking that were made possible by the large institutions' resources, said Dick Bove, an analyst with Rafferty Capital Markest who has five decades of experience in the industry. 

"Breaking up the banks is such a bad idea for Americans," he wrote in a note to clients, "that it is hard to imagine they will do this."

Trump, according to Bove, was likely to trying to appeal to an audience of community bankers he was meeting with on Monday when he reiterated his campaign-trail comments calling for a "21st-century version" of Glass-Steagall. 

While such a move might have bipartisan support in Congress, though how far it would go is uncertain. The repeal of Glass-Steagall under former President Bill Clinton was harshly criticized during the financial crisis, when critics said it helped fuel outsize speculation on a collapsing mortgage market and eventually forced the U.S. government to shell out billions in bailouts to protect the U.S. financial system. 

Earlier this year, Democratic Sen. Elizabeth Warren and three other senators reintroduced a bill seeking to reinstate some of the old law's provisions. On Trump's team, a key economic adviser, former Goldman executive Gary Cohn, suggested in April that the country's biggest financial institutions should separate consumer and investment businesses, and Treasury Secretary Steven Mnuchin, also a Goldman alum, said in his January confirmation hearing that he would support a "21st-century version" of Glass-Steagall but not going back to the original version of the law. 

Bankers, for their part, are confused by the lack of specifics. 

"I heard one of my peers say every time he hears about 21st-century Glass-Steagall, he asks everybody around him, 'What the heck does that mean?'" Morgan Stanley CEO James Gorman told analysts in April, speculating that such a proposal might involve "ring-fencing" commercial and investment operations. 

"There is and should be zero appetite for reinstituting Glass-Steagall itself, obviously, and I'm happy to see that that seems to be the prevailing view," Gorman added. "But whether we move to this ring-fenced model, I'm pretty confident we can deal with that here at Morgan Stanley."

JPMorgan, faced with questions on the matter from analysts, reiterated its position that the bank's diverse operations were an asset during the crisis.

"There is strength in the way the company operates that can't be discounted," CFO Marianne Lake said on an April earnings call. "I can't give you specific reasons to not continue to monitor the situation, but it doesn't feel consistent with the rest of the objectives of the administration."

Not only does a mandated breakup run the risk "of increasing the cost of banking products to consumers and slowing the delivery of new products to the market," Bove said, "advocates of shrinking the size of American banks never consider what this means in international markets.  It creates a vacuum into which large foreign banks will compete."

Still, a "profound lack of knowledge" in the U.S. about how banks work means anything can happen, Bove said.

At Citigroup (C - Get Report) , which drew 43% of first-quarter revenue from consumer banking and more than half from the division that includes trading, CEO Michael Corbat remains convinced his company's so-called universal model is the right one. 

"The administration is focused around trying to harmonize regulations, focused around trying to take things away that are either duplicative or don't really add value," he told analysts on an earnings call last month. "And I have yet to have anybody really explain to me what value there is in terms of either a reinstatement of Glass-Steagall, which in itself is strange, or what 21st-century Glass-Steagall is."

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