The federal government’s debt is fast approaching the $14.3 trillion ceiling that Congress put in place last year, sparking a furious debate in Washington about whether the cap should be raised or bigger budget slashes made instead.

Last week senior staff at ratings agencies Moody’s and S&P said US policymakers need to tackle the country’s debt situation very soon to avoid a change in credit outlook.

Equally worrying are states and municipalities in the US facing massive budget shortfalls, which has led star analyst Meredith Whitney to predict up to $100 billion in municipal defaults in the coming years — a figure disputed as exaggeration by many of her competitors.

So how real is the risk that the US could lose its AAA status with lenders, or even default?

“The likelihood of a restructuring of US sovereign debt is zero,” says MF global currency and fixed income analyst Jessica Hoversen. “As for a downgrade, while it’s theoretically possible, it is still extraordinarily unlikely.”

Hoverson points out that all measures of credit risk are relative and, as S&P noted in a report, “the US dollar is the world’s most used currency . . . depreciation and global rebalancing have not materially affected this privileged position.”

US debt has more than doubled since 2002 and increased by greater than 10 percent in each of the last three years.

With economic growth still soft and unemployment levels near historic highs, serious discussions of a downgrade in the US credit rating or even a default or restructure — once topics confined to the ranting of fringe bloggers — are increasingly showing up in mainstream media and research reports.

The status of the buck as the global benchmark does not appear likely to change soon: Both Portugal and Spain have been cited as potentially requiring bailouts similar to the ones already in place for Ireland and Greece, with the future of the EU a more pressing concern for investors than problems in the US.

Meanwhile, Japan, which this week announced it would buy bonds issued by the ECB to cover the cost of the Irish bailout, has a national debt that is now more than double its GDP.

Even China, with massive reserves at its disposal, is still years away from turning the yuan into a viable reserve currency, according to most market observers.

While the dollar is still king, the US will be able to issue debt and conduct trade in its own currency — protecting the nation’s balance sheet from external pressures.

So for now it seems Uncle Sam’s IOUs are still the safest on earth — even if they are losing luster.