Sen. David Vitter (R-La.) (L) and Sen. Sherrod Brown (D-Ohio) (R) speak during a press conference announcing the details of "Too Big to Fail" legislation on April 24 in Washington, DC. (Win McNamee/GETTY IMAGES)

A pair of lawmakers want the nation’s largest banks to sock away more capital to guard against a fresh economic downturn, making them less dependent on government bailouts.

Five years after the banking system nearly imploded and three years after Congress passed far-reaching measures to improve it, two senators have put forth an effort to rein in megabanks.

Sens. Sherrod Brown (D-Ohio) and David Vitter (R-La.) introduced legislation Wednesday that aims to end what the markets perceive as an implicit guarantee that the government will rescue big banks if they run into trouble. That implied backing has given firms a green light to engage in risky activities that pose a threat to the financial system, the lawmakers said.

Banking industry groups immediately called the legislation unnecessary, saying that the big banks have taken major steps since the financial crisis to bolster their finances and that any additional requirements would only hinder their ability to lend.

The legislation “presents Wall Street megabanks with a clear choice: Either have enough of your own capital to cover your own losses or downsize until you are no longer a risk to taxpayers,” Brown said at a news conference announcing the bill Wednesday.

Brown and Vitter are calling for institutions with more than $500 billion in assets — JPMorgan Chase, Bank of America and Citibank— to have capital equal to 15 percent of their assets. Banks with at least $50 billion, such as Fifth Third and BB&T, would have to set aside 8 percent. Community banks, those below the $50 billion threshold, would be exempt because they typically have large reserves, lawmakers said.

The bill also presses banks to use the most dependable form of capital, common equity — money raised from the sale of common stock and retained from profit.

That is a much stricter standard than the capital proposal of global banking regulators on the Basel committee , who include what is known as risk-weighted assets in their calculations. Risk weighting requires banks to hold more capital against assets deemed risky and less capital against those considered less risky.

“We’re focused on truly loss-absorbing capital and moving away from this hyper-complicated, risk-weighted system that is very easy to game,” Vitter said. If he and Brown get their way, the United States would not adopt the Basel committee’s capital rules.

The lawmakers are tapping into a popular sentiment among regulators and economists who say the Dodd-Frank financial regulatory law did not go far enough to end the “ too big to fail” problem. The law gave the Federal Deposit Insurance Corp. the authority to wind down failed firms and instituted so-called living wills, a blueprint for how banks could be resolved.

Veteran bank regulators, such as FDIC Vice Chairman Thomas Hoenig and Federal Reserve governor Daniel Tarullo, are among those in support of stricter rules to ensure taxpayers are not left on the hook for another bailout.Hoenig wants to outright break up the banks, whereas Tarullo would prefer big banks hold more long-term debt to ward against losses.

Even with those powerful allies, Brown and Vitter may have a difficult time getting their bill through Congress. The chairman of the Senate banking committee, Tim Johnson (D-S.D.), has said regulators should be allowed to finish implementing Dodd-Frank. A committee aide said the chairman looks forward to viewing the details of the bill.

The ranking Republican on the committee, Mike Crapo of Idaho, recently told Bloomberg that regulators, not lawmakers, should set capital standards. On Wednesday, he declined to comment on the bill.

Critics say Brown and Vitter are simply seizing on populist outrage with little regard for the detrimental impact the legislation would have on the financial industry. Increasing capital requirements to the level the bill suggests would significantly reduce the ability of banks to lend to businesses, said Rob Nichols of the Financial Services Forum, a trade group representing the largest banks.

“It would be ill-advised to take any lending capacity out of the economy — and away from the customers and businesses banks serve — as it’s struggling to get back on its feet,” he said.