THE TRUMP administration is more than halfway through the 120-day review of financial regulation that the president ordered on Feb. 3. Time flies! And the hot new idea is, well, an old idea: the Glass-Steagall Act, a Depression-era law which, until its repeal in 1999, separated federally insured commercial banks from risk-taking investment banks. Some say the law’s repeal helped lead to the panic of 2008 and that reinstating it would stabilize Wall Street more than the convoluted Dodd-Frank law enacted in 2010. Gary Cohn, the Wall Street veteran who heads the National Economic Council, expressed a willingness this month to consider a 21st-century Glass-Steagall, in keeping with certain vague 2016 campaign remarks by Donald Trump and a line in the Republican platform.

What that might mean in practice is still anyone’s guess. A bipartisan group of senators including Elizabeth Warren (D-Mass.) and John McCain (R-Ariz.) seized on Mr. Cohn’s remark to reintroduce their long-standing proposal to restore Glass-Steagall pretty much as it was in the 20th century. Thomas M. Hoenig, vice chairman of the Federal Deposit Insurance Corp. (FDIC), has a more nuanced plan that would allow commercial and investment banks to exist within the same overall corporate structure, but with clearly separate capital and management so taxpayers faced no exposure to the riskier investment side’s losses. Such a “ring-fencing” plan has been adopted in Britain; a key component is a tough capital requirement — 10 percent — for the commercial bank, as an added protection against systemic risk and taxpayer bailouts. Mr. Hoenig’s proposal calls for a similar buffer.

An irony of the situation is that some of today’s behemoth “universal” banks got that way because at the height of the crisis the government encouraged consolidation between, say, Bank of America and the failing Merrill Lynch investment bank. It would take tremendous effort — by legislators and bankers — to split them up again. The actual causal link between the repeal of Glass-Steagall and the financial crisis is a matter of great dispute, however, because the investment firms whose failures triggered the panic, Bear Stearns and Lehman Brothers, had never been subject to the law.

What they lacked was sufficient capital to weather a crisis; accordingly, beefed-up capital throughout the financial sector is what’s essential to protect against another meltdown, wherever it might originate. It so happens that Dodd-Frank, despite its undue complexity, has been fairly successful in forcing banks to build capital. This is why the Federal Reserve found last year that the U.S. banking system could withstand “a severe global recession with the domestic unemployment rate rising five percentage points.” In fact, Wall Street’s main complaint is that excessive capital requirements are forcing them to restrict lending — though FDIC data show that bank lending grew 5.3 percent in 2016 while the industry made healthy profits.

No doubt the financial sector would love to steer President Trump’s Dodd-Frank rewrite in the direction of relaxed capital requirements. Whatever else a 21st-century Glass-Steagall turns out to mean, it must not be that.