How does fiscal policy work

The rise of long-term interest rates since the beginning of the year has drawn notice. Niall Ferguson notes that he predicted the rise while Paul Krugman, relying on textbook Keynesianism, did not:

Of course, Mr Krugman knew what I meant. “The only thing that might drive up interest rates,” he acknowledged during our debate, “is that people may grow dubious about the financial solvency of governments.” Might? May? The fact is that people – not least the Chinese government – are already distinctly dubious. They understand that US fiscal policy implies big purchases of government bonds by the Fed this year, since neither foreign nor private domestic purchases will suffice to fund the deficit. This policy is known as printing money and it is what many governments tried in the 1970s, with inflationary consequences you do not need to be a historian to recall.

Over at Econlog Arnold Kling also notes the rise:

Greg Mankiw reports that the yield curve is steep, meaning that long-term interest rates have risen. In my view, this is perfectly rational, and it shows that the short-run effect of the fiscal stimulus is negative, as Jeff Sachs predicted.

This is all based on a Keynesian type of macro analysis. As we know, most of the stimulus spending does not take place until next year and beyond, so the short-run gains are puny. On the other hand, the big increase in the projected deficit creates the expectation of higher interest rates, which raises interest rates now. These higher interest rates serve to weaken the economy.