Deep within the Treasury Department sits a once-secret plan written by the Obama administration that could lead to the first-ever default on U.S. debt. Bond traders are worried that Donald Trump's Treasury secretary may have to use it.

The U.S. government will reach its statutory limit on borrowing some time in October, the Congressional Budget Office estimates. The Trump administration has asked Congress to raise the ceiling before then, but it is running into the same complications the Obama White House encountered: lawmakers, mostly Republicans, who want to use the debt limit as leverage for controversial policy changes.

Treasury Secretary Steven Mnuchin has said there are "plans and backup plans" to keep the government solvent through September. Bond traders suspect he is referring to preparations made in 2011 in case the Obama administration had to prioritize payments on government securities over other obligations. The Treasury chief got fresh hope that Congress may raise the debt limit before leaving for its August recess after Senate Majority Leader Mitch McConnell delayed the break by two weeks.