Until recently, the big threats to the Dodd-Frank financial reform law came from Republican lawmakers, who have vowed to derail it, and from banks and their lobbyists, who are determined to retain the status quo that enriched them so well in the years before, and since, the financial crisis. Now, the Obama Treasury Department has joined their ranks.

In an announcement on Friday afternoon — the time slot favored by officials eager to avoid scrutiny — the Treasury Department said it intends to exempt certain foreign exchange derivatives from key new regulations under the Dodd-Frank law. These derivatives represent a $4 trillion-a-day market, one that is very lucrative for the big banks that trade them.

A loophole in the law — which the bankers and their friends, including the administration, fought for — allows the Treasury secretary to exempt the instruments. The arguments in favor of exemption, beyond a desire to please the banks, were always unconvincing. They still are. The Treasury Department has asserted that the exempted market is not as risky as other derivatives markets, and therefore does not need full regulation.

That claim has been disputed by research, but even if it were true, it would be a weak argument. For instruments to be relatively safer than the derivatives that blew up in the crisis, necessitating huge bailouts, hardly makes them safe. Worse, dealers could probably find ways to manipulate the exempted transactions so as to hedge and speculate in ways that the law is intended to regulate.