WASHINGTON — Presidential candidate Sen. Bernie Sanders, I-Vt., is already causing headaches for the country's biggest banks, just days after announcing his bid to take on Hillary Clinton for the Democratic nomination.

Sanders, who describes himself as a democratic socialist, introduced legislation Wednesday to break up the largest financial institutions deemed "too big to fail." Both the bill and his candidacy are considered long shots, but his presence on the campaign trail will likely contribute to larger progressive pressures pushing Clinton to the left, particularly during primary season.

"From now until the nominating convention, Hillary Clinton's campaign is going to be focused on protecting her left flank, which suggests that the financial services sector as a whole is in for a year's worth of negative headlines," said Isaac Boltansky, a policy analyst at Compass Point Research & Trading.

The Too Big to Fail, Too Big to Exist Act would require the Financial Stability Oversight Council to compile a list of financial institutions considered "too big to fail," which would need to be broken up within one year. The list would be required to include, at a minimum, JPMorgan Chase, Bank of America, Citigroup, Goldman Sachs, Wells Fargo, Morgan Stanley, State Street and Bank of New York Mellon, along with other institutions deemed too risky by regulators.

"I fear very much that the financial system is even more fragile than many people may perceive. This huge issue cannot simply be swept under the rug — it has got to be addressed," Sanders said at a press conference Wednesday. "If an institution is too big to fail, it is too big to exist and that is the bottom line."

This is the third time the Vermont lawmaker has introduced legislation to break up the largest financial institutions with Rep. Brad Sherman, D-Calif., though this most recent effort has attracted fresh attention in light of Sanders' bid for the White House.

"We're in the exact same room we were in two years ago. There are only two differences. One is that the big banks have gotten even bigger. … The second change is that for some reason there are a lot more of you here today than there were two years ago," Sherman quipped at the press conference.

Sanders declined to discuss details of the campaign or Clinton on Wednesday, though he did highlight his longstanding criticism of several close advisors to Clinton's husband, former President Bill Clinton, in prepared remarks.

"I was one of the leading opponents of the efforts of Alan Greenspan, Robert Rubin and Larry Summers, who all told us how wonderful it would be if we deregulated Wall Street back in the 1990s," he told reporters.

Sanders noted, for example, that he fought against the Riegle-Neal Act, which made it easier to bank across state lines and the repeal of the Glass-Steagall Act, which separated commercial banking from riskier activities — two key changes to the system that spurred the growth of some of the country's top institutions today. Though he does not serve on the Banking Committee in the Senate, Sanders was previously a member of the financial services panel in the House.

"Clinton is not going to have to defend every part of her husband's administration, but there are going to be some inevitable, uncomfortable parallels," said Edward Mills, an analyst at FBR Capital Markets.

Even if Sanders' politics are considered too far left to ultimately win the Democratic nomination, his strong stances on banking and other economic issues are likely to influence the broader conversation. So far the Democratic field remains fairly narrow, with Martin O'Malley, the former Maryland governor, still weighing a run. O'Malley wrote in a March op-ed that he supported breaking up the big banks and reinstating the Glass-Steagall Act — essentially aligning himself with Sanders and Sen. Elizabeth Warren, D-Mass., who has proposed legislation bringing back the Depression-era law.

"If Martin O'Malley does declare, he's more aligned with Sanders, and Hillary Clinton is either going to have to move closer to them, or there's going to be some distinctions that could be a problem within the party," said Mills.

The Sanders bill already has early support from several progressive groups, in addition to the Independent Community Bankers of America.

"The Sanders-Sherman legislation would require policymakers to identify and downsize firms whose failure would pose a catastrophic threat to the financial system or U.S. economy without substantial taxpayer assistance," Camden Fine, president and chief executive of ICBA, said in a statement Wednesday.

Meanwhile, critics of the effort were quick to dismiss the bill.

"I fully expect Senator Sanders' third attempt to break up banks to have the same impact as the previous two: zero. Breaking up banks would be incredibly disruptive in the short run and anti-competitive in the long run," said Tony Fratto, a partner at Hamilton Place Strategies and a former Bush administration official, said in a statement. "The presumed financial stability benefits [of making banks smaller] just aren't there — in the last crisis, many of today's largest banks became a safe harbor in the storm."

Still, despite some early enthusiasm for Sanders — he raised an eye-popping $1.5 million within a day of announcing his candidacy — observers cautioned that the biggest player in the Democratic primaries remains Warren, who has vowed she won't run.

The Massachusetts Democrat is a firebrand for progressives concerned about big banks and efforts to roll back the Dodd-Frank Act, and her influence on the campaign will be tangible, even without joining the fight herself.

"I'm still a believer in the idea that the sheer gravitational field of the Elizabeth Warren brand was enough to pull the Democratic primary conversation to the left, which has left me with the sense that Sen. Sanders isn't as impactful on financial services issues as Warren, even though she's not in the race," said Boltansky.

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