Here's who gets hit hardest on Wall Street if Glass-Steagall comes back

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Breaking up the banks might make campaigning politicians look good, but it won't help bank investors much.

That's the big takeaway from a Keefe, Bruyette & Woods report issued over the weekend, which looks at the prospects and ramifications of bringing back Glass-Steagall, legislation that put a wall between commercial and investment banking but was done away with in the late 1990s.

Glass-Steagall unexpectedly jumped back into the political conversation when Republicans referred to their desire to bring back elements of the legislation in July as part of their campaign platform. Democrats doubled down, and said they want to restrict Wall Street banks more. Not everyone is happy about the shift , and KBW analysts say it's not a good deal for most on Wall Street, including stockholders.

"Investors should not view the reinstatement of Glass-Steagall as a potential way to unleash value in large banks," they wrote. "A Congressional approach to breaking up the banks would not be based on economic value creation, but be based on the politics of applying penalties to the largest banks."

Re-instituting Glass-Steagall as law could include fully splitting investment and commercial banking businesses, limiting commercial banks' subsidiaries to underwrite only government-issued securities, prohibiting investment banks from taking deposits, and forbidding interlocking roles between investment and commercial banks for directors or managers. In other words, it would undo a lot of things that have changed in the banking business since the financial crisis.

For investors, what little upside is to be found would likely come through investing in smaller banks and brokerage firms that are "single product focused," the analysts wrote. But even smaller banks with more than 3 percent of their revenue coming from brokerage activities would be forced to make changes, they said.

The biggest impact to banks would be felt at JPMorgan Chase , Citigroup and Bank of America , the analysts wrote. Wells Fargo would be forced to divest mortgage banking activities, but those operations are smaller for the West Coast bank than for the others.

Virtually every Wall Street major bank would feel pain from Glass Steagall's return; even Morgan Stanley and Goldman Sachs would have to shed deposit funding businesses, the analysts wrote. For investment banks in particular, reverting to their precrisis state would represent a substantial shift in strategy. Goldman Sachs, in particular, has been beefing up deposits as it looks to disrupt the traditional brick-and-mortar banking business online.

Citigroup, JPMorgan Chase and Morgan Stanley declined to comment; other banks did not immediately respond to CNBC requests for comment.

"If there is a push for a renewed Glass-Steagall, we believe there is likely to be renewed downward pressure on the size of these companies," analysts wrote.

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