One of the frustrating things about covering American politics from a vaguely left-liberal perspective is that many of the left-left theories turn out to be true, or true enough. You try to point out to the street protesters and tenured Marxists that things are more complicated than Noam Chomsky and the late Paul Sweezy would have you believe, and, all too often, it turns out they aren’t much more complicated. The richest 0.1 per cent really is getting richer and richer while most Americans see their living standards stagnate. The C.I.A. really did torture people in secret prisons overseas, and the N.S.A. has just received authorization to carry on gathering all of your phone records. The big banks and corporations really do run Washington—or, at least, that’s how it seems on this chilly December day.

Late Thursday night, by a vote of 219–206, the House of Representatives passed a spending package that will finance most of the federal government until next September. In so doing, it also did the big banks a favor, by rolling back some of the restrictions that had been placed on their riskier activities following the financial crisis of 2008. The bill passed despite the opposition of Nancy Pelosi, the Minority Leader; many liberal Democrats; and even some Republicans—although most of the latter objected to financing the federal government rather than slipping in a favor to Wall Street.

Why was a piece of financial deregulation tacked onto a spending bill? If you have to ask that question, you don’t follow Washington very closely. Since spending bills usually, but not always, get passed, corporate lobbyists routinely get them festooned with giveaways to various business interests. This practice isn’t new. In fact, some of the disastrous pieces of financial deregulation from the nineteen-nineties, which allowed the banks to engage in some of their riskiest operations with little oversight, were enacted in this way.

The details of the particular measure that passed on Thursday are a bit arcane, but the principles involved aren’t. After taxpayers were forced to bail out the big banks in 2008, the Obama Administration, Congress, and the various regulatory agencies tried to make sure that it wouldn’t happen again. The authorities forced the banks to hold more capital in reserve, banned them from engaging in some speculative activities, and forced them to move some of their other activities to subsidiaries that aren’t guaranteed by the federal government in the case of a blow-up. It was this last reform that the measure smuggled into the spending bill sought to overturn.

According to some accounts, the actual language of the measure was drawn up by a lobbyist for Citigroup. According to others, Jamie Dimon, the chairman of JPMorgan Chase, who is widely lauded as the king of Wall Street, personally called some legislators and asked them to vote for it. President Obama and the White House, scandalously enough, had already been squared away. Hours before the vote, Josh Earnest, Obama’s press secretary, said that the President had some reservations about the spending bill, but added that passing it would remove the threat of another government shutdown and provide “the kind of certainty that’s important to our economy.” It was left to Pelosi, Elizabeth Warren, and other congressional liberals to try to rally opposition. “It’s more billions of dollars in subsidies for Wall Street. It’s morally reprehensible,” Sherrod Brown, the Ohio Democrat, told reporters. “They’re saying government bailouts are back.”

That’s an oversimplification, but it’s along the right lines. During the real-estate bubble that lasted from 2003 to 2007, many of the big banks drove themselves to the brink of bankruptcy by building up huge exposures to an overvalued market through the issuance of derivative securities known as credit-default swaps. At megabanks like JPMorgan and Citi, these activities were carried out under the umbrella of the Federal Deposit Insurance Corporation, which insures bank deposits. The F.D.I.C.’s guarantee helped to enable the banks to get cheap financing and make big short-term profits, because lenders could be assured that if anything were to go wrong, the federal government would step in with a bailout, which is precisely what happened.

The Dodd–Frank financial-reform bill, which was passed in 2010, contained an amendment that forced the megabanks to move all of their swaps operations out of the main bank and into uninsured subsidiaries. There is legitimate debate about whether this provision, which was proposed by Blanche Lincoln, the former Democratic senator from Arkansas, really overcomes the incentives problem. If an uninsured subsidiary of JPMorgan were facing a loss of tens of billions of dollars, say, it could well spark a run on other parts of the bank by creditors and counterparties, which would force the government to intervene even though it didn’t have any legal obligation to do so.

In any case, though, under Lincoln’s provision, the banks (and their creditors and counterparties) were forced to reckon with the possibility that their derivatives-trading subsidiaries would be allowed to fail. That could limit risk-taking, raise financing costs, and reduce the banks’ profits, which explains why they opposed the reform so vigorously, and, eventually, managed to get part of it rolled back. “It is because there is a lot of money at stake,” Simon Johnson, an M.I.T. economist who is an expert on banking and regulation, told the Washington Post’s Wonkblog. “They want to be able to take big risks where they get the upside and the taxpayer gets the potential downside.”

As Johnson also pointed out, the importance of this particular measure shouldn’t be exaggerated. It doesn’t repeal the Volcker Rule, which restricts proprietary trading, and even after it has been passed, some types of swaps trading will still have to be spun off into subsidiaries. (The Wall Street Journal’s Washington Wire has a good explainer.) But for the banks, and for Dimon in particular, Thursday’s vote amounts to a big victory—one that confirms all the money they spend on lobbying and campaign donations in Washington doesn’t go to waste. And what does it say about American democracy? I fear you already know the answer to that one.