(Reuters) - A downgrade to the U.S. sovereign credit rating could open up a new world of pain for the dollar.

Already reeling from low interest rates, slow economic growth, and foreign investors eager to diversify away from U.S. assets, the loss of AAA status could cement the view the dollar is no longer the safest harbor in a troubled world. The risk of a downgrade remains real even after Washington's $2.1 trillion budget savings deal, since it fell well short of the $4 trillion Standard & Poor's said would be enough to support the AAA rating with a stable outlook. That it took so much drama to produce such a limited round of cuts has disappointed investors who had grown weary of fiscal weakness during budget crises in the euro zone countries. "A debt ceiling raised plus downgrade equals weak dollar," said Jonathan Lewis, founding principal of Samson Capital Advisors in New York, which manages assets of $7 billion. "Not only would a double AA rating be a concern for international investors, but the fiscal imbalances would not be a good reason to buy the dollar." For years, the dollar has acted as the world's reserve currency, an international store of value for central banks. However, the fact the other safe-haven currencies are gaining at the expense of the dollar suggests investors' views may already be changing, perhaps in anticipation of a downgrade or at least a tough fight to hang on to AAA. Over the last month, the dollar plummeted 6 percent against the Swiss franc and about 4 percent against the yen. "Being the world's reserve currency seems incongruous with a double-A rating," said Barclays Capital in a research note. The only consolation, perhaps, is that the dollar has risen more than 1 percent over the past month against the euro, though only because the euro zone itself is under the gun over fiscal problems of Greece and Italy.