[…] Besides, Democrats could easily interpret (and should, vindictively) the warning from S&P as a call for higher taxation.
Precisely. S&P is commenting on the inability of said gubmint to actually do anything and most definitely not on the underlying capability of the United States economy to produce growth and/or sustain a marginally higher tax rate necessary to retire enough of the debt to keep the entirely mythical bond vigilantes at bay.
But, yeah, why does anyone alive care what S&P or Moody’s et al. says? Serious question. They may as well manufacture high quality buggy whips for all their relevance post-meltdown; there is no greater indictment of the lack of serious change to our financial system than this.