The U.S. is nearly $20 trillion in debt, a number that has almost doubled under President Barack Obama. If Donald Trump is elected president, the nation's massive pile of IOUs will keep on growing and then some.

Already the self-proclaimed "king of debt" — a declaration the Republican nominee made on CNBC in May — Trump promised Thursday to use the low-interest environment as a means to rebuild the national infrastructure.

"This is a time to borrow and borrow long term," he told CNBC during a discussion on how he would finance the many projects he wants to undertake, such as rebuilding airports and bridges and upgrading the military.

Absent from Trump's myriad criticisms of Obama has been anything about debt. The president entered office with the U.S. owing the world $10.6 trillion, and that has swelled to $19.4 trillion, according to the Treasury Department.

While the debt load has gone from 87 percent to 104 percent compared to gross domestic product during the Obama administration, it has remained largely manageable thanks to rock-bottom interest rates courtesy of the Fed.

By way of comparison, the U.S. paid $383 billion in interest on that debt in 2009; in 2015 that number grew to $402 billion, just a 5 percent increase despite an 83 percent surge in total debt, Treasury figures show.

During that time, the Fed had kept its interest rate target anchored near zero, a policy that began during the 2008 financial crisis and didn't move during the recovery until the central bank hiked a quarter-point in December.