Lawmakers on Capitol Hill are on the brink of letting their nation slide over the so-called fiscal cliff - leaving senators to fume that nobody does dysfunction better than Congress.

US citizens would likely agree. The country's elected representatives have long been derided as hapless slackers. But this year, with a fiscal crisis now staring at people like a deer in the headlights, it's different.

The 112th Congress is being seen as the least effective in decades.

On Sunday, during a rare holiday-week Senate session, they cemented that reputation, when the mood among many lawmakers inside the Capitol reflected the wintry conditions outside: cold, dark and somewhat unforgiving.

With just hours before the US economy was to suffer half a trillion dollars in tax hikes and spending cuts in 2013, senators shuttled between the chamber and their caucus lunches, unable to agree on a path forward to avoid a crisis that has been nearly two years in the making.

Many of them said they had little knowledge of the horsetrading that was going on behind closed doors.

"I haven't heard there's a deal," Senator Barbara Boxer accurately pointed out to AFP.

Senator Barbara Mikulski, the longest-serving woman in the history of Congress, sounded nearly apoplectic, describing the gridlock as "one of the lowest points" in her 35 years as a lawmaker.

"I have lived through 9/11, when we were attacked by outside forces. I've lived through the impeachment of a president, which was so enormously difficult. But this is what we're doing to ourselves," she said.

Negotiations involving Vice-President Joe Biden and Senate Republican leader Mitch McConnell appeared to offer the last hope for avoiding the across-the-board tax increases and draconian cuts in the federal budget that will be triggered at the start of the new year because of a deficit-reduction law enacted in August 2011.

A jolt from the financial markets could also prod the parties, as it has occasionally in the past.

AFP-Reuters