The debt of the U.S. federal government is almost $19 trillion, yet a number of economics are calling for increased borrowing to finance increased program spending. How will this debt be repaid and in view of the increased debt how will current government operations be sustained?

One theory is that in times of low interest, it's good to borrow to stimulate programs that stimulate hiring and so on... it has worked in the past.

Debt is a 20 volume set of big fat books. There's a wealth of information out there just by googling 'US debt.' This is not the right place for people to write them again.

The idea is, that the government does not worry about paying it back. They assume that will be some other political persons problem.
They only worry about providing things for the public and getting re-elected.

They believe for foreign policy and to help control other nations, they must give them money.
They are even talking about free health care, free university and more which all costs money.

A much higher tax, of about 50 to 60 percent of earnings is about the only possible way to do it.

Other nations of course merely defaul on the loans at some point and start with bad credit but no debt.