WASHINGTON — The United States government is financing its more than trillion-dollar-a-year borrowing with i.o.u.’s on terms that seem too good to be true. But that happy situation, aided by ultralow interest rates, may not last much longer. Treasury officials now face a trifecta of headaches: a mountain of new debt, a balloon of short-term borrowings that come due in the months ahead, and interest rates that are sure to climb back to normal as soon as the Federal Reserve decides that the emergency has passed. Even as Treasury officials are racing to lock in today’s low rates by exchanging short-term borrowings for long-term bonds, the government faces a payment shock similar to those that sent legions of overstretched homeowners into default on their mortgages. With the national debt now topping $12 trillion, the White House estimates that the government’s tab for servicing the debt will exceed $700 billion a year in 2019, up from $202 billion this year, even if annual budget deficits shrink drastically. Other forecasters say the figure could be much higher. In concrete terms, an additional $500 billion a year in interest expense would total more than the combined federal budgets […]