Sanders’s bill would bar financial institutions from holding assets, derivatives and other forms of borrowing worth more than 3 percent of the entire U.S. economy. That would cap their size at $584 billion in today’s dollars.

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The legislation would force federal regulators to break up six different Wall Street firms — JPMorgan, Bank of America, Citigroup, Wells Fargo, Goldman Sachs and Morgan Stanley — as well as insurance giants such as Prudential Financial and MetLife. Collectively, the targeted firms hold more than $13 trillion in assets, according to Sanders aides.

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Despite its unlikelihood of passing in the near term, the measure could become a marker for Democrats seeking support from the party’s progressive voters, much like a single-payer, universal health-care system has become.

Sanders touts the plan as a way to prevent a repeat of the financial crisis of a decade ago, when banks on the edge of collapse were large enough that their failures would rock the fundamentals of the global financial system. In response, the federal government extended the banks massive loans, a move largely credited with blunting the crisis but that also gave government funding to wealthy individuals at a time when unemployment was soaring and thousands were losing their homes.

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“We spent huge amounts of money bailing them out, and they are significantly larger now than they were back then,” Sanders said in an interview. “It’s time we return to that discussion, especially now for the 10th anniversary" of the crash.

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In response to the crisis, Democrats narrowly passed a broad banking law that was meant to ensure that “too big to fail” banks took steps to ward off failure, subjecting the largest firms to more stringent restrictions aimed at limiting their risk.

The law, signed in 2010 by President Barack Obama, had 16 separate chapters and ran more than 2,300 pages long. Sanders’s measure runs seven pages and, instead, goes after the size of banks, arguing firms of that size pose an inherent risk to the economy.

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The senator, who identifies as a Democratic socialist, and his supporters say the firms should be broken up to prevent future rescues, while critics say Sanders is advancing an unpractical bill with no chance of being enacted.

“This legislation cuts to the heart of the matter, by putting a size cap on the largest highly leveraged firms. The size cap is simple, straightforward, and transparent,” said Simon Johnson, an economist at the Massachusetts Institute of Technology who served as chief economist of the International Monetary Fund and supports the bill. “This measure will bring us closer to full and fair competition in the financial system, where a few megabanks currently predominate.”

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Four of the six biggest banks are on average 80 percent bigger than they were when they started receiving bailout funding about a decade ago, according to Sanders aides, as many of the largest financial firms acquired distressed banks during the crisis. JPMorgan, which acquired Bear Stearns in 2008 at the urging of the federal government, has grown by about 60 percent, to $2.53 trillion, according to the company’s public disclosure forms.

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Sanders’s plan was criticized by some analysts and allies of the financial industry, who pointed to dramatic improvements in the capital cushions that banks now hold to ward against collapse. Dodd-Frank, Obama’s banking law, put new capital requirements into effect for the largest financial institutions.

The Financial Services Forum, a D.C.-based lobbying firm that represents large financial services companies, said in a statement that legislation “to address so-called too big to fail” banks risked ignoring the role these banks play in supporting the global economy. The group represents banks targeted by Sanders’s legislation, such as Bank of America, Citi, JP Morgan and Goldman Sachs.

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“To have a large, strong economy that supports households and businesses big and small, you must have large, strong, global banks,” said Kevin Fromer, president and chief executive of the Financial Services Forum, in an email. “The banking industry and governments around the globe have made enormous strides during the past decade to ensure that large banks are safe and sound and that no institution is too big to fail. Policymakers must neither ignore the progress that has been made nor the essential role of large financial institutions in our economy.”

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Jim Kessler, co-founder of the centrist think tank Third Way, said Democrats should instead focus on expanding capital in rural areas where he said small-business lending has stalled.

“No one has ever lost an election going against the big banks. But I’m not sure anyone has won an election going against the big banks,” Kessler said. (In an interview, Sanders pointed to the ads Trump ran in the 2016 presidential election promising to take on Wall Street.)

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Even some broadly sympathetic to Sanders’s efforts to restrain the size of Wall Street banks think the legislation is too ambitious. Thomas M. Hoenig, who recently stepped down as vice chair of the Federal Deposit Insurance Corporation that regulates the banking industry, said it would already be a very difficult to get enough support even to pass a far less stringent cap on banks' size.

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Hoenig pointed out that Republicans were joined by more than a dozen Democratic senators in dismantling parts of the 2010 Dodd-Frank banking bill, an effort that many Republicans thought did not go far enough.

“Several Democrats just voted to ease capital standards on two of the largest banks,” Hoenig said. “So who is going to pass this law?”

Sanders had already called for breaking up the biggest Wall Street firms, but this new bill offers a new mechanism for doing so. Previously, Sanders had called for the Financial Stability Oversight Council to identify and break up “too big to fail” institutions in addition to supporting the reinstatement of Glass-Steagall — the 1930s law keeping commercial and investment banking separate. The new approach sets a clean cap on a financial institution’s size.

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Sen. Sherrod Brown (Ohio), the top Democrat on the Senate Banking Committee, has also proposed legislation capping the size of U.S. banks. Sanders’s bill uses a broader measure than Brown’s bill to capture the size of the bank, which means Sanders’s legislation would force more banks to have to shrink by a greater margin, said Johnson, the MIT economist. (Brown’s bill came before the Federal Reserve recently began publishing a new estimate of banks’ “total exposure,” which factored in borrowing that does not apply on financial balance sheets.)

But the bill may also form the cornerstone of future Democratic banking legislation, as the party has moved toward a raft of populist left-leaning ideas, including Medicare-for-all and free tuition at public universities.

Robert Hockett, a professor at Cornell University who specializes in banking issues and helped draft Sanders’s “too big to fail” legislation, said the measure may more easily garner public support and offers a significant improvement on previous banking legislation.

“Like Obamacare, Dodd-Frank is very long, very nuanced, and very difficult to explain to people,” Hockett said, referring to the Affordable Care Act. “It’s so much easier to explain Sanders’s bill.”