The US hit the debt ceiling set by Congress today, forcing the Treasury Department to start taking extraordinary measures to keep the government paying its bills and escalating pressure on Capitol Hill to avoid a catastrophic default.

The battle lines for the high-stakes fight have already been set. Hardline Republicans, who have enormous sway in the House because of the party’s slim majority, have demanded that lifting the borrowing cap be tied to spending reductions.

The White House countered that it will not offer any concessions or negotiate on raising the debt ceiling. And with the solution to the debt ceiling drama squarely in lawmakers’ hands, fears are growing that the partisan brinksmanship could result in the nation defaulting on its debt for the first time ever – or coming dangerously close to doing so.

Treasury Secretary Janet Yellen wrote a letter to House Speaker Kevin McCarthy today, informing him that the nation’s outstanding debt is at its statutory limit of $31.4 trillion and that the agency will implement extraordinary measures so it doesn’t default on its debt, which would have enormous consequences on the US economy, global financial stability and many Americans.

She said the measures would last through June 5.