Hillary Clinton said nothing on Wednesday night that should derail her considerable chances of winning the presidency on November 8. But if she wins, one simple promise she repeated over and over again could come back to haunt her reelection bid in 2020.

“I also will not add a penny to the debt,” Clinton said toward the beginning of her final presidential-debate performance. She made a similar pledge two more times that night, and it’s a line she has used before on the campaign trail. It’s a short-hand reference to the fact that although she has proposed hundreds of billions in new federal spending for infrastructure, paid family leave, education, and other items, she would pay for those investments by raising an equal or greater amount in revenue through higher taxes on the wealthy and corporations.

The problem for Clinton is that even if she succeeds in enacting all of her fiscal policies—a humongous “if”—the national debt will still go way up during her time in office. The national debt now stands at about $19.7 trillion, and if policymakers wanted to keep it there, they’d have to approve hundreds of billions of dollars in immediate spending cuts or tax increases—neither of which Clinton is actually proposing. “She had to have misspoke. The alternative would be absurd,” Dean Baker, a liberal economist who co-founded the Center for Economic and Policy Research, said in an interview Thursday. “Clearly, she is going to add to the debt.”

Politicians often use the terms “debt” and “deficit” interchangeably, but they refer to two different figures. The annual budget deficit, which stood at about $587 billion in 2016, is the gap between what the federal government spends and what it collects in tax revenue in a given year. The public debt, as the Treasury Department defines it, is the accumulation of annual deficits—the amount of money the government borrows by selling Treasury securities.

Deficits have gone up and down over the years, but the debt has only gone in one direction—up—and it will continue to do so under the next president, whether it is Clinton or Donald Trump. “Whoever is president will actually be adding $9 trillion to the debt over the next 10 years unless they make changes,” said Maya MacGuineas, president of the Committee for a Responsible Federal Budget, a nonpartisan fiscal watchdog group. “So it won’t be policies that they put in place” that will cause an increase, “but the current trajectory is that we’ll borrow $9 trillion over the next 10 years.” Much of that $9 trillion will come from so-called mandatory spending on programs like Medicare, Medicaid, and Social Security, as well as the 16 percent of the annual budget spent on defense. Neither Clinton nor Trump is proposing to significantly curtail those programs, and Trump has vowed to spend more on defense.

“For a candidate to say they have a plan that wouldn’t add a penny to the debt, they’d actually need a plan to save $9 trillion,” MacGuineas said. “She doesn’t have that, and no one could actually even be expected to have that. We are going to add to the debt.

“What she’s saying in budget-speak is she’s not going to add more to the debt than we are already on track to borrow,” she said.

That’s what Clinton’s campaign says as well. Aides on Thursday said she was simply talking about her “pay-as-you-go” approach to fiscal policy, which they characterized as a middle ground between the austerity budgets proposed by Republicans in Congress and the budget-busting tax cuts that Trump has championed. Still, the clear-cut debt promise is a questionable one from a candidate famous for her attention to detail. For support, her campaign highlighted an analysis of her economic plans by the Committee for a Responsible Federal Budget. Yet even that study found that Clinton’s policies would add $200 billion to the debt over a decade, in large part because she has not specified the business taxes she said she would raise.

Still, Clinton would be on stronger footing if she had used the word “deficit” instead of “debt.” President Obama, for example, can credibly claim to have presided over a sharp reduction in the annual budget deficit even as the national debt has nearly doubled during his tenure. And while the current campaign hasn’t focused on policy, it’s easy to see how Republicans in two or four years could turn Clinton’s pledge not to add a penny to the debt into the next iteration of George H. W. Bush’s “Read my lips: No new taxes,” or Obama’s health-care assurance, “If you like your plan, you can keep it.” The TV ad juxtaposing Clinton’s words and the national debt clock practically writes itself.

For liberal Democrats, the bigger concern about Clinton’s line is not the particular wording but that she felt the need to make that kind of promise in the first place. They have argued that the fixation of many Republicans and centrist fiscal hawks on the debt and deficit is misplaced and that the economy is still too weak to support budgetary belt-tightening. “On the more general point of deficits, the country’s problem since the crash in 2008 has been deficits that are too small, not too large,” Baker wrote on Thursday.

Clinton is running well to the left of her husband’s 1990s agenda in many areas, but her vow to keep a lid on deficits is, to some progressives, a relic from that era. Baker concedes, however, that the rhetoric of fiscal prudence remains popular with voters. “We have to change the conversation on this,” he told me. “It’s unfortunate she’s chosen to reinforce it.”

Clinton got a pass from the Progressive Change Campaign Committee, which cheered her answers on fiscal policy, particularly her stated support for expanding Social Security benefits rather than cutting them. As for her promise on the debt, spokeswoman Kaitlin Sweeney said Clinton did a good job responding to a question from moderator Chris Wallace that was “right-wing.” “She didn’t talk about it in terms of austerity. She didn’t talk about it in terms of reducing the size of government,” Sweeney said.

MacGuineas’s complaint, on the other hand, is that neither candidate is bold enough when it comes to tackling the nation’s ever-rising national debt. Clinton, she said, “has made a promise she will not be able to keep, but she has also shied away from making a promise she should try to keep, which is to bring that planned borrowing down.”