The bigger the fiscal deficit is the more Treasuries investors - including the Federal Reserve - need to buy. At the same time, the more Treasuries that have to be sold, the highest the interest rate all else equal... until something snaps (or unless an stock market crisis forces the Fed and investors to monetize/park cash in Treasurys).
This was, in a nutshell the grim message from the IMF's latest Fiscal Monitor Report, which warned that the US would be the only country with growing debt levels over the next 5 years.
What the IMF did not elaborate on, however, is that in many countries, such twin deficits have resulted in a debt crisis. So, picking up where the IMF left off, Deutsche Bank conducted an analysis which found that "the deteriorating fiscal and external situation for the United States have increased the probability of a US debt
crisis by 7 percentage points, from a historical average below 9% to a level around 16%."