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Fitch Ratings said in a statement that the AAA rating the US now bears would be in jeopardy if Congress can't get a deal done before the deadline to raise the borrowing limit.
"If the debt limit is not raised in a timely manner prior to the so-called 'x date' Fitch would review the US sovereign rating, with potentially negative implications," Fitch said. "We have previously said that prioritizing debt service payments over other obligations if the limit is not raised - if legally and technically feasible - may not be compatible with 'AAA' status."
There are three major credit ratings agencies. If the US were to slip close to a debt ceiling crisis, their ratings on the US debt could slip and make it more costly for the federal government to issue debt. In 2011, after the US brushed up against the debt ceiling deadline, S&P lowered its rating of US debt, sending shockwaves throughout financial markets.
Congress has 12 days in session during the month of September to make a move on the debt ceiling before the Treasury Department's self-imposed deadline.
One option that has been floated in previous debates is the idea of "prioritization" - that the government could stop other payments and focus on paying interest on its debt to try and preserve faith in the bond market. Fitch said, however, that likely wouldn't help if it reconsidered its rating.
"In Fitch's view, the economic impact of stopping other spending to prioritize debt repayment, and potential damage to investor confidence in the full faith and credit of the US, which enables its 'AAA' rating to tolerate such high public debt, would be negative for US sovereign creditworthiness," Fitch said.
Fitch noted that there are disagreements within Congress that need to be solved in order for the debt ceiling to be raised. Despite their majority in both chambers, Republicans will likely need some Democrats to come on board with a clean hike of the limit, since hardline conservatives have opposed such a hike.