WASHINGTON — In US political lore it is considered bad form to try to steal the limelight from a presidential rival during his party's celebratory nominating convention.

But President Barack Obama's Democrats are refusing to be held hostage by tradition and plan an all-out assault on Republican rival Mitt Romney as the nail-biting 2012 White House race whirls towards a decisive finale.

In US presidential elections, raucous conventions are held a couple of months before polling day at which the candidates are formally nominated and party grandees and rising stars try to whip up support with stirring speeches.

Republican faithful are converging on Tampa Bay, Florida for a four-day convention that will culminate on Thursday with Romney and running-mate Paul Ryan accepting the nomination to take on Obama in the November 6 election.

The run-up to the convention has been overshadowed by scandalous remarks on rape from a Republican congressman, so the Democrats don't want to let Romney off the hook by giving his campaign total freedom to reboot.

"This election is going to be close and we are not taking a single vote or a single day for granted," Obama campaign spokeswoman Jennifer Psaki told AFP.

"There is ample precedent for candidates and incumbents campaigning during the convention."

Obama will hit the campaign trail on Tuesday and Wednesday in the key swing states of Iowa, Colorado and Virginia, while First Lady Michelle Obama will appear on Wednesday night on CBS's "Late Show with David Letterman."

But the most brazen attempt to deflate the Republican convention balloon is a direct gate-crash by Vice President Joe Biden, who will hold campaign events in Florida on Monday and Tuesday, including at least one in Tampa.

With a rousing speech to black leaders in July after Romney was booed by the same audience, and pinpoint jabs at the multi-millionaire challenger's business record and tax secrecy, Biden has relished his role as Obama's attack dog.

He has pressed Romney to release more tax returns and taken pleasure in recalling the that candidate's father, who lost the 1968 Republican nomination to Richard Nixon, disclosed 12 years of his returns, unlike his son's two.

"Folks, this is not your father's Republican Party. And this is not even Mitt Romney's father's Republican Party," he told voters on Wednesday, seeking to paint the current crop of Republicans as extreme and out-of-touch.

Biden will issue the rallying cry in Charlotte, North Carolina on September 6 as Democrats revice a tradition of both the vice presidential candidate and the presidential candidate addressing the final night of their convention.

Republicans played down the Biden challenge in Tampa and sought to drive the narrative back to the flagging US economy and their portrayal of Obama as a president who has failed to deliver for struggling working-class Americans.

"Vice President Biden's inevitably gaffe-filled speech will only serve to further highlight the contrast voters face between the Romney-Ryan ticket's bold solutions for fixing our economy and an administration that has failed to live up to the promises they made to voters four years ago," Republican National Committee spokesman Tim Miller said.

The announcement of the Letterman show appearance by Michelle Obama comes as the New York Times reported that the main US television networks would not broadcast live Monday's convention speech by the challenger's wife, Ann Romney.

A study by the Pew Research Center showed Thursday that the 2012 presidential race is even more bitter than usual but neither side appears to be gaining substantial benefit from the barrage of negativity.

Locked together in national polls, Obama has a slim lead in some key swing states but the race is considered neck-and-neck.

"The static nature of the narrative reflects a fundamental reality of the race in recent months," the study said.

"There has been no signature event that might cause the media to re-evaluate the storyline or perception of either candidate."