It's actually an obsolete leftover, and you could (this came up the last time) argue that it violates the 14 Amendment's clause on the debt:
The validity of the public debt of the United States, authorized by law, including debts incurred for payment of pensions and bounties for services in suppressing insurrection or rebellion, shall not be questioned.
By raising the question of whether or not the debt incurred previously will be paid, the debt limit violates this clause, or so it was argued last time around.
The Democrats, last time they controlled the Congress, passed and got signed pay as you go rules in 2010. Why did the Republicans oppose this bill? Because it said any new law which increased the deficit had to be offset by either spending cuts or tax increases. They objected, of course, to the idea that the offset could be a tax increase.
But if this is in place why is the deficit so big? Because it applied to new laws, not existing ones. This past Congress didn't do much in the way of new bills; the House of Reps, occupied by Tea Party extremists, spent most of its time grandstanding on abortion bills that were never going to see the light of day anyway. What little time they had leftover they spent grandstanding uselessly about Bernanke or how they would never ever ever raise a tax on anyone ever.
In other words, proudly beating their chests over how they weren't going to do a thing, since their job actually does consist of debating and doing the deals that get new laws passed. But if you're not willing to do a deal, you have nothing to do as a politician.
Except grandstand.
And so here we are now, and those same Tea Party extremists are still grandstanding, since none of the new people elected in November take office until after this debt limit debate is over.
As for the ratings agencies, they'll do as they please and no one will care, except it'll make a nice headline. The bond market will decide what happens, not them. As always.
