By Richard Allen Greene
BBC News, Washington
No-one expected the fight for control of the US Congress to be a gentlemanly affair, but as races come down to the wire, even experts are surprised at how vicious some of the advertising has been.
And one commercial, in the southern state of Tennessee, has set "a new low in American politics", according to John Geer, who studies negative campaigning.
The advert is a satirical attack on Harold Ford Jnr, the Democratic candidate for Senate, and it has been accused of "race-baiting" - or playing the race card - by the nation's best-known civil rights group, the NAACP, among others.
It shows a series of people offering tongue-in-cheek reasons to vote for Mr Ford - such as a hunter saying, "Ford's right, I do have too many guns", and a mock-sleazy type asking, "So he took money from porn producers. Who hasn't?"
But where critics say the advert crosses the line is when a blonde woman - with no clothes visible - says "I met Harold at the Playboy party".
As the spot ends, she winks and says breathily: "Harold - call me!"
"That is when the ad breaks new ground, and unfortunately in a bad way," says Mr Geer, a political science professor at Vanderbilt University in Tennessee and the author of In Defense of Negativity.
Running in the state where the white-supremacist Ku Klux Klan was founded in the wake of the Civil War, the commercial is intended to stoke "old fears about racial equality", he says.
"Ford's right - I do have too many guns," the attack runs
The Playboy character is apparently a reference to Harold Ford's attending a Super Bowl party sponsored by the magazine last year.
Mr Ford has made no apology for going to the party, saying: "I like football and I like girls."
The commercial created such a storm of protest that it was pulled early from the airwaves by the Republican National Committee - which paid for the piece but said that, under America's arcane campaign-finance laws, it had no input into the content.
The party chairman, Ken Mehlman, denied that the commercial was racist.
"As someone who is extraordinarily sensitive to it, I don't believe it was. At the same time there are good people on both sides who believe otherwise."
Vital to democracy
Mr Geer is among them.
He says he was surprised the Republicans would air such an advert when their candidate held a slight lead in polling.
"I wonder if the Republicans have other data that suggest they are in trouble. I had always thought they would play the race card only if they were behind," he adds.
His analysis reflects a central thesis of his book - negative campaigning works.
And he argues that negative campaigning - criticising your opponent rather than boasting about your own merits - plays an important role in democracy.
"Negative appeals are more substantive. Anyone can say 'I am for freedom and apple pie.' When you attack, you have to be specific.
"The negative ads have much more documentation than the positive ads. Democracy really requires negativity if you want to hold somebody accountable."
But, he adds: "You can't just make these ads without some nugget of truth."
Setting record straight
That is exactly what Brooks Jackson fears is happening.
He is the director of FactCheck.org, a non-partisan project of the Annenberg Public Policy Center at the University of Pennsylvania.
He and his team of researchers have gone into overdrive this campaign season, debunking campaign commercials from North Carolina to California.
"In my considerable experience covering US campaigns, I can't think of a more negative one. Both sides are at it," he says.
He cites a Republican attack advert accusing a Democrat of voting to "study the sex lives of Vietnamese prostitutes in San Francisco" rather than funding sickle-cell research, and a Democratic commercial accusing a Republican of profiting from changes in Medicare.
FactCheck labelled the first piece "distasteful" and "misleading", while the evidence for the second was "flimsy at best".
Mr Jackson says he was particularly irritated by a commercial created by VoteVets.org which is being run against several Republican senators, including George Allen of Virginia.
It shows a soldier firing at two dummies, one wearing a "Vietnam-era" vest and another wearing "modern body armour".
Bullets penetrate the old vest but not the new one, and then the soldier accuses Sen Allen of voting against giving troops the armour they need.
The problem, Mr Jackson says: "There has never been a vote on body armour."
Fine print at the end of the commercial cites a Senate vote, but Mr Jackson said the amendment in question "did not include a word about body armour, and not a word was said about body armour during the debate".
VoteVets has defended the accuracy of the commercial.
Evidence it works
Mr Jackson says it is common to find attacks that rely on some fact - but which lead to an incorrect assumption.
"It is rare that we find an ad that is absolutely made up," he says.
But, he says, there was only limited risk in taking liberties with the truth.
"The only way an attack ad can blow up is if it can be proven that you're lying - and everyone gets the message."
So for the moment, the attacks are certain to continue.
"The political consultants believe that this sort of stuff works," Mr Jackson says. "And there is evidence that in certain circumstances, it does."