Robust economic growth has helped push the U.S. budget deficit down to the lowest level since 2008, marking the sharpest turnaround in the government’s fiscal position in at least 46 years.

The shortfall of $483.4 billion in the 12 months ended Sept. 30 was 2.8 percent of the nation’s gross domestic product of $17.2 trillion over the same period, according to data compiled by Bloomberg using Commerce Department figures. The figure peaked at 10.1 percent of GDP in December 2009.

“That’s what happens when the government is holding itself back on spending and the economy is improving,” said George Goncalves, head of interest-rate strategy at Nomura Holdings Inc. in New York. “The question is, is that as good as it gets or will the deficit continue to shrink?”

The narrowing budget deficit has bought time for lawmakers to solve long-term threats to the economy such as the cost of retirement benefits. Gregory Valliere, chief political strategist for Potomac Research Group, said the fiscal relief may be short lived as austerity-weary lawmakers eventually boost spending on defense and other programs.

“I can see the beginnings of a pendulum shift away from fiscal restraint,” he said.

The Congressional Budget Office in August predicted the deficit will shrink further this fiscal year, to 2.6 percent of GDP, before rising to 2.9 percent in the presidential election year of 2016. Before the fourth quarter of 2008, the last time the deficit-to-GDP share reached 2.8 percent was in April 2005, the data show.

Debt Auctions

The reprieve is enabling the government to reduce the amount of debt sold in the short term.

The Treasury yesterday said its borrowing this quarter will decline to the least for the October-December period since 2007. The department made the projection in Washington ahead of its quarterly refunding announcement tomorrow.

Scott Brown, chief economist at Raymond James & Associates Inc., said the fiscal improvement has muted the political debate over the budget ahead of today’s midterm congressional elections. “The bigger problem with the budget is really the long-term pressure, and has to do with the retirement of the baby-boom generation,” Brown said.

The CBO’s baseline budget predicts spending on mandatory programs, including Social Security and Medicare, will expand by 72 percent to $3.63 trillion in 2024 from $2.11 trillion this year. That would raise the budget deficit as a proportion of GDP back to 3.6 percent, according to the CBO.