Scare-mongering ads, voter registration forms dumped in the trash and misleading statements on the stump: the list of dirty tricks sullying the US presidential election is seemingly endless.

With the high-stakes race culminating with voting Tuesday, experts warn that the unfortunately typical attempts to keep a rival's supporters from the polls or sway voters with flat out lies could end up deciding the outcome.

"If an election is close those kinds of things can matter," said Kathleen Jamieson, director of the Annenberg Public Policy Center of the University of Pennsylvania.

"We've had the chastening experience of 2000. And 2004 was close as well."

Some of the uglier tactics could end up backfiring, Jamieson said, citing a DVD sent to voters in swing states claiming to have uncovered nude photos of President Barack Obama's mother in a bondage magazine.

"The problem when that sort of sleaze is distributed is you'd have to be a hardcore partisan to believe it. It's not like they changed anyone's vote," Jamieson told AFP.

"You run the risk that you alienate more people than you mobilize."

False and misleading statements on the stump and in advertisements run the risk of alienating voters by undermining a candidate's trustworthiness.

But given the "abysmal" level of political knowledge among most voters, the payoff often outweighs the risk, said editor Brooks Jackson.

"The level of deception or the degree to which candidates stray from the facts or reality has more to do with the closeness of the election and the perception of how high the stakes are than just about anything else," he said.

"I think we can move the needle a little bit, but only by increasing the level of public knowledge. Not by shaming politicians. There's too much at stake."

Republican White House hopeful Mitt Romney recently came under fire from both Democrats and the heads of General Motors and Chrysler for ads that falsely implied an automaker rescued by Obama is now shipping American jobs to China.

The Obama campaign has also told its share of whoppers, including claiming that Romney plans to outlaw all abortions when he has clearly stated his support for exceptions in the case of rape, incest and a threat to the life of the mother.

Fringe presidential candidate Randall Terry, an anti-abortion activist, is able to sidestep decency laws and air gruesome ads depicting aborted fetuses by turning it into protected political speech.

One ad declares that "a vote for Obama helps Muslims murder Christians and Jews" after showing images of decapitated and bloodied bodies.

Most of the dirtiest tricks -- many of which are illegal -- are carried out by third parties. The list of known incidents is long:

- Voters in the crucial swing states of Florida and Virginia reported calls from fraudsters telling them they could cast their ballot by phone.

- The FBI is investigating letters sent to voters in Florida, many of whom were Republican, falsely claiming that election officials were questioning their citizenship.

- The Republican National Committee fired a voter registration firm long dogged by allegations of fraud after reports surfaced in September that it submitted forged forms in Florida, Colorado and Nevada.

- Another Republican contractor, Colin Small, was fired last month after he was caught tossing registration forms from Democrats in Virginia into a dumpster.

- The Democratic National Committee fired Texas staffer Stephanie Caballero after an undercover video showed her giving advice on how to vote in two states.

- The son of Virginia Congressman Jim Moran, a Democrat, resigned after the same muckraker caught him on tape talking about how to cast ballots under someone else's name.

- Billboards warning "voter fraud is a felony" in predominantly African American and Hispanic neighborhoods in the battleground state of Ohio were taken down last month after complaints they were aimed at intimidating voters.

Meanwhile, a conservative group called True the Vote has sparked fears of voter intimidation with its vow to train thousands of volunteers to challenge any "suspicious" people at the polls as they seek to ferret out voter fraud.

While tossing out voter registration forms is a serious problem, many dirty tricks don't have much of an impact on the outcome, according to elections expert Rick Hasen of the University of California, Irvine.

"What is likely to have a larger impact is changes in laws, especially laws passed by Republican legislatures, which have the potential to moderately suppress Democratic turnout such as cutting back on early voting, making registration more difficult and voter ID laws," said Hasen.