How much significance the $20 trillion breakthrough will have depends on perspective.
On one side, there's the argument that the debt and deficits don't matter now because interest rates are low and the deficit only amounts to 2.9 percent of gross domestic product. On the other side are the hawks who believe that while financing costs are cheap at this point, the current trajectory ultimately will be unsustainable.
In a report released this week, the Congressional Budget Office warned that current spending is on track to take debt held by the public to nearly $25 trillion by 2027, or 89 percent of GDP. The deficit is the amount of money the government spends each year against what it takes in, while the debt is the cumulative amount the government borrows to finance its operations.
"Such high and rising debt would have serious negative consequences for the budget and the nation," the CBO said in a report, adding that "the likelihood of a fiscal crisis in the United States would increase."
When that tipping point starts to happen is unclear.
While the $20 trillion threshold could generate some attention when it is broken, more attention could come in 2018 when the debt-to-GDP ratio starts "pivoting," said Ernie Tedeschi, policy economist at Evercore ISI.
"2018, according to what the CBO has been telling us over the past couple of years, is when you start to see on one hand the real burden of demographics on entitlement spending on things like Social Security, Medicare and Medicaid come into play, and when you see the recovery in tax revenues from the recession ... be for the most part complete," Tedeschi said.
The Trump factor
The political climate also complicates the picture.
The debt climb during Obama's term came with economic growth that clocked in at only around 1.6 percent. Trump has promised stimulus spending that many economists believe will accelerate growth, but also could worsen the debt burden. (Trump has pledged that growth plus spending cuts elsewhere will counterbalance spending increases, and therefore the debt burden will not increase.)
At the same time, the Federal Reserve's policy of keeping interest rates low during the past eight years kept debt costs down. But the Fed plans to get more aggressive in pulling back from its ultra-low rates stance this year, which likely will drive up the government's debt costs.
"Donald Trump has said that entitlements are off the table and he doesn't want to cut defense spending," Pento said. "In a world of skyrocketing debt and deficits and central banks that will be selling, who's left to buy? No one is really talking about how high interest rates could go and how artificially depressed they are if you look at them in historic terms."
Even with low rates, the U.S. paid $432 billion in debt service in fiscal 2016 and already has shelled out $139.1 billion for the new fiscal year that started in October. Total debt financing costs during the Obama years came to about $3.3 trillion, according to the Treasury.
One rule of thumb is that each quarter-point rate hike translates into $50 billion more in debt service. So rising yields, even under the Fed's relatively muted forecasts, could add hundreds of billions to debt costs.
"Some of the numbers become so eye-popping that we can't ignore them. But if we wait too long, it will be too late to make these choices responsibly," MacGuineas said. "If we do wait until we're forced by a bad economy or our creditors to make changes, those changes will be so much more painful and the economy will go through a much more massive downturn."