Allegations of voter fraud are not only difficult to prove, they're likely to prompt bipartisan debate.
And how voter fraud complaints were handled is one the issues emerging from the growing furor over the firing of eight U.S. attorneys.
Some of the attorneys who were dismissed were accused of failing to aggressively pursue Republican complaints of voter fraud. The White House concedes it passed along such complaints to the Attorney General's office.
John McKay, the former U.S. attorney in Seattle, said he was accused of mishandling voter fraud when he interviewed at the White House for a federal judgeship. The specific question: why he had mishandled the investigations into voter fraud connected to Washington's very close 2004 gubernatorial election. That election was won by a Democrat.
He did not get the appointment. Three months later, he was terminated.
"I was surprised because the case was so clear," McKay told All Things Considered in a recent interview.
McKay says his office thoroughly reviewed every allegation, and every piece of evidence submitted in a state court case to overturn the election.
"We concurred with the state trial court judge that there was no evidence — and let me just emphasize, zero evidence — of election voter fraud in that election," he said. "And so we, in consultation with the voting rights section of the Department of Justice, and fully documented with the Department of Justice, concluded that there should be no further federal investigation on the election."
For example, he says, there was evidence that convicted felons voted even though they're not allowed to under state law. But it turns out that the state mailed them their ballots, making it difficult to prove that the felons willfully broke the law.
In a similar case, former U.S. Attorney David Iglesias also concluded, after an investigation, that voter fraud complaints in New Mexico were "not criminally prosecutable."
Barnard College professor Lorraine Minnite says in the complicated, sometimes messy, world of elections, things are rarely clear-cut.
"When we actually look for evidence that voters are criminally corrupting the electoral process, we just don't find a lot of evidence of that," she said.
She says proven cases of fraud are extremely rare. For example, Republicans alleged widespread double voting in recent elections in both New Jersey and Connecticut, but investigations found no sign of fraud. Minnite, along with many Democrats, thinks that Republicans exaggerate the extent of fraud to drum up support for proposals to restrict access to the polls, such as a requirement that voters show ID.
But Republicans say there is evidence of fraud, and that the potential for fraud is a very serious problem.
"I believe there's at least an element of fraud in every single state," said Rep. Steve King (R-IA).
He said there's a lot of evidence. He also says there's no question a federal law encouraging people to sign up to vote when applying for a driver's license has allowed some non-citizens onto the registration rolls. You are automatically registered unless you tell the clerk not to register you.
"And that opens up the polls to people who don't have a right to vote here," he said.
He cited an audit in Utah that found that several hundred non-citizens had registered to vote there. He said 14 had actually cast ballots. Other Republicans note the conviction in Missouri of three people who submitted false voter applications there last year. And they cite Justice Department figures showing 87 convictions for ballot fraud since 2002.
There were 122 million votes in the 2004 election.
Republicans say Democrats don't want to acknowledge the extent of the problem because they fear any solution will cost them votes.
Doug Chapin of the non-partisan electionline.org says no one really knows how much voter fraud is out there.
"The suspicions frequently outstrip the evidence," he said. "What we don't know is whether or not the suspicion is unfounded or the evidence is as yet uncovered."
He adds that there isn't even an agreed-upon definition of voter fraud, further complicating any debate over whether or not enough is being done to stop it.