I declined to read the linked piece because my time is valuable and I have no interest in reading anything put out by the Mercatus Center. I have now read the piece at your insistence and it confirms that you don't know your ass from your elbow. For starters, the United States is not an emerging market so the applicability of the working paper to the United States is, well, it isn't applicable at all. Second, even assuming it were applicable it doesn't say anything about the creditworthiness of the United States at present. What we do have available to us is the interest rate the United States government has to pay to borrow money. Typically, folks look to the 10 year note as a gauge. To put it simply, where the market believes there is some risk that the United States will default on its repayment obligations, interest rates are higher and where the risk is lower, the rates are lower. Here is a chart of rates dating back to 1962:
As you can see, rates are quite low by historical standards and are substantially lower that in 1983 when President Reagan wrote the letter linked above. To pretend that the United States is less creditworthy now when it can borrow at an interest rate of about 3.18% as compared to over 10% in 1983 you have to be either stupid or dishonest. You choose.