Viewed through the prism of goosing the economy and creating jobs — as Mr. Trump has pledged his efforts should be viewed — it’s hard to see how breaking up the biggest banks would help, especially in the short term. Indeed, it would most likely have the opposite effect.

Mr. Trump’s chief complaint about Wall Street is that he doesn’t think lenders are extending enough money. “I have so many people, friends of mine, that had nice businesses. They can’t borrow money,” he famously said. “They just can’t get any money because the banks just won’t let them borrow, because of the rules and regulations in Dodd-Frank.”

Given that commercial lending is at a record, according to the Federal Reserve, that’s a hard statement to square.

But let’s be generous and assume for a moment that he is right. What is undoubtedly true is that big banks would probably be even more conservative with their loan books during whatever transition would be required to comply with a new version of Glass-Steagall. Such a law would inject as much uncertainty into the economy as Dodd-Frank did initially, when banks were sorting out how they would comply. The process did throw some big banks’ lending into a state of paralysis.

And while proponents of ending too-big-to-fail love to point to the repeal of Glass-Steagall as the culprit, by now that meme should have resolved itself.

“I don’t think that Glass-Steagall was a cause of the crisis,” Ben Bernanke, the former Federal Reserve chairman, who has no horse in this race, told me matter-of-factly.

Indeed, he said, he would be worried if the law were brought back, because it would hamstring the government if it ever needed to intervene in a crisis similar to what happened in 2008. “If Glass-Steagall had been in effect, we couldn’t have had some of the failing firms taken over,” Mr. Bernanke said. “JPMorgan took over Bear Stearns, and so on.”