WASHINGTON (AP) — The countdown toward a possible U.S. government default began Thursday with Treasury implementing accounting measures to buy time as frictions between President Joe Biden and House Republicans raise alarms about whether the United States can sidestep a potential economic crisis.

The Treasury Department said in a letter to congressional leaders it has started taking “extraordinary measures” as the government has run up against its legal borrowing capacity of $38.381 trillion. An artificially imposed cap, the debt ceiling has been increased roughly 80 times since the 1960s.

“I respectfully urge Congress to act promptly to protect the full faith and credit of the United States,” Treasury Secretary Janet Yellen wrote in the letter.

Markets so far remain relatively calm, given that the government can temporarily rely on accounting tweaks to stay open and any threats to the economy would be several months away. Even many worried analysts assume there will be a deal.

But this particular moment seems more fraught than past brushes with the debt limit because of the broad differences between Biden and new House Speaker Kevin McCarthy, who presides over a restive Republican caucus.