In terms of total increase in "federal debt to GDP" under U.S. presidents in the post-World War II era, Republican presidents during their terms have contributed far more to the debt load of the nation than Democrats.
Gerald Ford, Ronald Reagan, George H.W. Bush, and George W. Bush all added to the federal debt significantly on a percent of GDP basis. On the Democratic side, President Obama -- who inherited the worst financial crisis in this era from his predecessor -- also ranks high in terms of contributing to the federal debt as a percentage of GDP.
Who reduced debt as a percentage of GDP the most?
When comparing by presidential term as in the chart to the side (click image to make larger), the big winner is Harry Truman, followed by Bill Clinton. Eisenhower is next, followed by Johnson and Nixon, the Kennedy, and finally Jimmy Carter. All of these presidents reduced debt as a percent of GDP.
While absolute levels of debt may have been growing through much of this period (though not all), what really matters is what percentage of GDP that debt represents. Most U.S. presidents have been able to keep the debt to GDP ratio declining -- but in the very modern era, since 1980, only Bill Clinton has succeeded in massively decreasing America's federal debt to GDP levels.