Under which presidential administration did the national debt triple, due to a combination of tax cuts and defense spending?

President Reagan

The national debt rose rapidly during the 1980s. President Reagan followed a combination of lowered tax rates (from 70% to 28% at the top income tax rate) and increased defense spending. At the same time, Democrats blocked cuts to social programs. This led to the tripling of the Gross Federal Debt, from $900 billion to $2.7 trillion. The debt continued to rise during the presidency of George H. W. Bush, but decreased at the end of Bill Clinton's presidency.