Also known as the debt limit, this is a law that limits the total amount of money the government can borrow to pay its bills.
This includes paying federal employees, the military, Social Security and Medicare, as well as interest on the national debt and tax refunds.
From time to time, the US Congress votes to raise or suspend the ceiling so that more borrowing can be done.
Trump’s addition of the condition to the deal was an unexpected requirement that made it difficult for right-wing Republicans, who often oppose increased government spending, to support the bill.
The debt ceiling was set to be raised in the coming months, regardless of the promised efficiency gains.
But by calling for it to happen now, some analysts say Trump is signaling the cost of his proposed tax cuts, while tightening border restrictions will test the limits. And he prefers the fight for promotion to take place before he takes office.
Democrats usually favor raising the debt ceiling without limits, but they are in no mood to help Republicans right now.
The risk for Trump is that the mini-rebellion sheds the cloak of invincibility he has worn since the election.
And that raises questions about how smoothly his party will be able to deliver on its campaign promises, which require legislation.
