Sandy Weill, populist hero? Photo: Adam Rountree/Bloomberg via Getty Images

On the most recent episode of The Newsroom, Sloan, the sharp-witted financial anchorwoman played by Olivia Munn, delivered a soliloquy on the merits of the Glass-Steagall Act. (Quick refresher: Glass-Steagall was a very old, very revered bit of financial regulation that prohibited commercial banks from engaging in risky investment activities, and that was repealed in the late nineties as part of a push to create the giant, too-big-to-fail banks we have today.)

Sloan, for one, was nostalgic for the days of Glass-Steagall:

It helped lead to the longest sustained period of economic growth in U.S. history. A 60-year expansion of the middle class, the largest increase in productivity, and the largest increase in median income. We also won WWII, put a man on the moon, and a computer in everyone’s lap.

The view that Glass-Steagall should be reinstated, and big banks broken up into smaller pieces, is generally seen by Wall Street types as a fringe view these days — a kind of loony left-wing nostalgia cooked up by people who simply don’t understand the modern capital markets.

But this morning, Sloan and the rest of the “break up the banks” brigade got an unlikely ally in the form of former Citigroup chairman Sandy Weill.

Weill, if you’ll remember, was the architect of the 1998 merger between Travelers Group and Citicorp that created Citigroup and led to Glass-Steagall’s repeal in the first place. (According to a 2010 Times profile, Weill was so proud of his role in creating a supermarket bank that he kept a self-portrait etched with the words “The Shatterer of Glass-Steagall” in his office.)

But Weill did a 180 on CNBC’s Squawk Box this morning, saying that he now believes big banks — like, presumably, Citigroup — should be broken up:

What we should probably do is go and split up investment banking from banking, have banks be deposit takers, have banks make commercial loans and real estate loans, have banks do something that’s not going to risk the taxpayer dollars, that’s not too big to fail.

Sandy Weill advocating for the reinstatement of Glass-Steagall is among the biggest flip-flops imaginable. (In political terms, it would be akin to Rick Santorum announcing he was becoming a GLAAD spokesman.) And when Weill called for a bank breakup this morning, CNBC’s incredulous anchors gave him a chance to walk back what they assumed was a spur-of-the-moment gaffe. But Weill didn’t budge:

I’m suggesting that they be broken up so that the taxpayer will never be at risk, the depositors won’t be at risk, the leverage of the banks will be something reasonable.

To some, Weill’s sudden about-face reeks of hypocrisy. Others are picking it over for evidence of ulterior motives. (Could Weill be trying to undermine his former protégé and another too-big-to-fail banker, JPMorgan CEO Jamie Dimon?)

But what seems clear is that breaking big banks up to avoid systemic risk and avert future taxpayer-funded bailouts, an idea whose most outspoken supporter was once Democratic Senate candidate Elizabeth Warren, is hitting a nerve among the very people who led the charge against Glass-Steagall in the first place.

That means that the debate about the soundness of having giant, FDIC-insured commercial banks attached to risk-taking investment banks has moved from the periphery to the mainstream. And it means that Wall Street megabanks will have to defend their size a lot sooner than they’d hoped.