It all started here: As a short-run strategy to reduce inflation and lower nominal interest rates, the U.S. borrowed both domestically and abroad to cover the Federal budget deficits, raising the national debt from $997 billion to $2.85 trillion. This led to the U.S. moving from the world's largest international creditor to the world's largest debtor nation. Reagan described the new debt as the "greatest disappointment" of his presidency. From 1975 through 2005 the Democrats increased Federal spending 9.9% and increased the Federal debt 4.2% during the same period the Republicans increased Federal spending 12.1% and the Federal debt 36.4%.
I think the most relevant statistic is the ratio of national debt to GDP. The higher the GDP, the better able we are to carry debt. Wikipedia has some interesting numbers: http://en.wikipedia.org/wiki/National_debt_by_U.S._presidential_terms
As I recall most of it went to increased "defense" spending. He basically drove the Soviet Union into bankruptcy trying to keep up with Star Wars and the like. It only took us an additional 15-20 years or so to follow on, depending on where you mark the bankruptcy of the US. Hank Paulson's "Give me seven hundred billion dollars or there will be tanks in the streets" extortion speech might be a good place.
Regan was an actor. He took on debt without having a plan to pay it back. He left it for George to deal with. George didn't know what to do with it except increase it. He left it to see if Clinton could deal with it. At this point it started spiraling out of control. Clinton couldn't stop it. The George II couldn't deal with it and he just spent like a teenage girl with her daddy's credit card. Now its Obama's turn and he isn't really doing anything to stop or at least slow down this debt spiral of death. Is it really surprising that the S&P gave the US a 'negative' outlook? I think it's long overdue.