Sens. Sherrod Brown (D-Ohiol) and David Vitter (R-La.) have been working on a bill to block the largest banks and financial firms from receiving federal subsidies for being deemed Too Big to Fail. On Friday, a draft version of that bill was leaked to Tim Fernholz of Quartz, much to Vitter's chagrin. So, what does the bill do?

Let's start with what it doesn't do: It doesn't break up the big banks. Rather, it focuses on how much capital they have to hold to protect themselves from disasters and would “prohibit any further implementation of” the international Basel III accords on financial regulation.

But let’s back up. Banks hold capital to protect against losses. The more capital they hold, the safer they are from crisis. As Alan Greenspan said after the financial meltdown, "[t]he reason I raise the capital issue so often, is that, in a sense, it solves every problem." (Here is Ezra Klein’s introduction to capital ratios.) The "ratio" in question is the amount of capital against the amount of assets. So, if a bank has $10 in cash and $100 in assets, its capital ratio is 1:10.

Regulators set minimum capital ratios for banks. A capital ratio is like any other ratio, with a numerator and denominator. Some amount of capital held goes on top, and some value of the assets the bank holds goes on the bottom. The Brown-Vitter legislation would significantly change both parts of that ratio.

This is where things get a bit wonky: Common equity is viewed as the best form of capital because it can directly absorb losses. Basel III puts more emphasis on using common equity than previous versions. There’s a baseline 4.5 percent buffer, which is supplemented by a 2.5 percent “capital conservation buffer.” In addition, Basel III also has requirements for categories of less effective forms of capital, grouped under Tier 1 and Tier 2, or “total capital.”

As for the denominator, Basel III has risk-weighted the assets held by the firms. Firms use models and ratings to determine an asset's risk. The riskier the asset, the more held capital needed in case of a loss. An asset rated as less risky requires less held capital. (You may remember the financial crisis involved both the ratings agencies and the financial sector getting these ratings very wrong for subprime mortgages.)

The Brown-Vitter proposal would not adopt Basel III. It would instead have a baseline of 10 percent equity in the numerator consisting solely of common equity. There are also surcharges for capital over $400 billion, which would cover all assets regardless of their risk-weighting. So there would be a significant increase in equity. The denominator would also increase, forcing banks to hold even more capital. This approach has much in common with the recent book "The Banker’s New Clothes," by Anat Admati and Martin Hellwig, and should be seen as a win for those arguing along these lines.

Though it might seem like a technicality, risk-weighting assets is as significant in this proposal as a higher capital ratio. Risk-weighting was introduced by the first Basel in the late 1980s, using broad categories. It evolved to, among other goals, encourage firms to build out their risk management teams. However, those teams often acted as regulatory arbitrage teams instead. Many people view the system as encouraging race-to-the-bottom regulation dodging, backward-looking strategies that reduce capital held in a bubble and techniques that use derivatives and bad models to keep capital ratios low.

Regulators are growing more critical, both domestically and internationally, about Basel III.  That regulation has several measures to address problems with risk-weighted assets, from adjusting the numbers used to requiring capital for derivative positions. But it is unclear how well these will work in practice.

Basel III has to be enacted by the banking regulators in the United States. The process began last summer (see a summary here). As Federal Reserve Governor Daniel Tarullo notes, regulators are expected to finish the Basel III capital rules this year and begin working on the rules for new liquidity requirements and other parts of Basel III.

It is interesting that the Brown-Vitter bill would replace, rather than supplement or modify, Basel III. Basel III has a leverage requirement that does similar work to the extra equity requirements Brown-Vitter recommends. That rule is only set at 4 percent, instead of 10 percent, but could be raised while keeping the rest of the Basel rules intact.

Because even those who want financial institutions to hold a lot more capital and less leverage may see a few downsides to abandoning Basel III. If firms go into Basel’s newly created capital conservation buffer, they can’t release dividends and are limited on bonuses. This, to use banking regulation jargon, is a way of requiring “prompt corrective action” on the part of both regulators and firms, who will normally drag their feet.

Basel III isn’t just capital ratios, though. Another important element is its new liquidity requirements. Liquidity here refers to the ability of banks to have enough funding to make payments in the short term, especially if there’s a crisis. Basel III includes a “liquidity coverage ratio,” which requires banks to keep enough liquid funding to survive a crisis.

Financial institutions have been lobbying against an aggressive implementation of Basel IIl’s liquidity requirements. They saw a small victory when some of the requirements were pulled back in the final rule in January. Brown-Vitter  would remove them entirely -- a remarkable win for the financial sector if the proposal passes.

(There are already some liquidity requirements made since the financial crisis, but they aren’t as extensive as Basel lll. And because they have evolved, it's unclear what would happen to them.)

Note that this bill is explicit in not breaking up the big banks, either with a size cap or by reinstating Glass-Steagall. Two months ago in the House, Rep. John Campbell (R-Calif.) also introduced a bill designed to end Too Big To Fail, which called for banks to hold special convertible debt instruments while also repealing the Volcker Rule. There’s been a lot of talk about conservatives becoming aggressive on structural changes to the financial sector, but so far there’s no evidence of this in Congress.

During the drafting of Dodd-Frank, Treasury Secretary Timothy Geithner argued against Congress writing capital ratios into law, preferring to leave it to regulators at Basel to find an internationally agreed-upon solution. Basel’s endgame is now coming into focus, and there needs to be a debate on how well it addresses our outstanding problems in the financial sector when it comes to bank capital. This bill means reformers might start to rally around the idea that dramatically increasing capital, as well as removing the emphasis given to measuring risks, is an important part of ending Too Big To Fail. Even if that means going against the recent Basel accords.