WASHINGTON, USA (AFP) — Former president Bill Clinton is stumping for Barack Obama in a new campaign ad, saying the president has a stronger plan to return the US to "full employment" than his Republican rival Mitt Romney.

In addressing the top issue on voters' minds -- the sour US economy -- the Democratic Party stalwart lashed out at Republicans in the 30-second spot for planning to "cut more taxes on upper-income people and go back to deregulation".

"That's what got us into trouble in the first place," said Clinton, who was in office from 1993 to 2001.

The video, in which the man some Democrats still call the "Big Dog" speaks throughout, is slated for broadcast in key states ahead of the November 6 election.

"This election, to me, is about which candidate is more likely to return us to full employment," said Clinton, 66.

"President Obama has a plan to rebuild America from the ground up, investing in innovation, education and job training. It only works if there is a strong middle class."

Romney, a multimillionaire former entrepreneur, says his business success qualifies him to save the US from unemployment, stagnating around eight per cent. Unemployment before the 2008 economic crisis was only five per cent.

When Obama was running against Clinton's wife Hillary in the 2008 presidential election, the former president criticised him for his lack of experience.

But the attacks were largely forgotten when Obama named Hillary Clinton his chief diplomat in 2009. And Bill Clinton is now regularly deployed by the Obama team to defend its platform.