Moody’s has warned that a shutdown would be credit negative for the US sovereign. This does not necessarily mean there will be a downgrade.

Though they said a short-lived shutdown would be unlikely to disrupt the economy, “it would underscore the weakness of US institutional and governance strength relative to other Aaa-rated sovereigns”. They pointed to greater political polarization that constrains US fiscal policy.

Fitch downgraded the US sovereign rating to AA+ in August citing “repeated debt-limit political standoffs and last-minute resolutions” and S&P downgraded their rating for the US to AA+ in 2011 following a government shutdown.

Other Aaa-rated countries include Germany and Canada.