The federal budget deficit for 2019 is estimated at $984 billion, a hefty 4.7 percent of gross domestic product (GDP) and the highest since 2012, the Congressional Budget Office (CBO) said on Monday.

The difference between federal spending and revenue has only ever exceeded $1 trillion four times, in the period immediately following the global financial crisis.

The deficit, which has grown every year since 2015, is $205 billion higher than it was in 2018, a jump of 26 percent.

The CBO has warned that the nation's debt is on an unsustainable path. Higher levels of debt increase borrowing costs, make it harder for the government to battle economic downturns and increase the share of future spending devoted to paying off interest costs.

Since President Trump took office, the GOP has passed a massive tax cut package that reduced revenue, while Democrats and Republicans have agreed to increase spending year after year.

Budget watchers note that the main drivers of the deficit, however, come from automatic spending programs such as Social Security, Medicare and Medicaid.

"Democrats and Republicans must be held responsible for the outrageous deficit reported today by the CBO," said Jason Pye, vice president of legislative affairs at the conservative advocacy group FreedomWorks.

"This unsustainable situation is only going to get worse," he added.

The final Treasury Department figures for the fiscal year, which ended on Sept. 30, will be published later this month and could include worse news.

Previous Treasury estimates projected the deficit for the year surpassing $1 trillion.