Two relevant points:
1) The statutes of most states require third parties registering prospective voters to submit ALL registration forms they receive, even those that are problematic.
2) Actual instances of illegal votes being cast as a result of registration fraud are extremely rare.
In a 2007 report titled "The Truth About Voter Fraud," New York University's Brennan Center for Justice stated, "We are aware of no recent substantiated case in which registration fraud has resulted in fraudulent votes being cast."
According to the Brennan Center:
There have been several documented and widely publicized instances in which registration forms have been fraudulently completed and submitted. But it is extraordinarily difficult to find reported cases in which individuals have submitted registration forms in someone else's name in order to impersonate them at the polls. Furthermore, most reports of registration fraud do not actually claim that the fraud happens so that ineligible people can vote at the polls.
http://brennan.3cdn.net/e20e4210db075b482b_wcm6ib0hl.pdf#page=6Summary
Fraud by individual voters is both irrational and extremely rare.
Many vivid anecdotes of purported voter fraud have been proven false or do not demonstrate fraud.
Voter fraud is often conflated with other forms of election misconduct.
Raising the unsubstantiated specter of mass voter fraud suits a particular policy agenda.
Claims of voter fraud should be carefully tested before they become the basis for action.
http://truthaboutfraud.org/documents/policy_brief_voter_fraud.html Voter fraud worries – about Acorn or anyone else – are unsupported by the facts, said experts on election fraud, who recall similar concerns being raised in several previous elections, despite a near-total absence of cases.
"There's no evidence that any of these invalid registrations lead to any invalid votes," said David Becker, project director of the "Make Voting Work" initiative for the Pew Charitable Trusts.
Becker should know: he was a lawyer for the Bush administration until 2005, in the Justice Department's voting rights section, which was part of the administration's aggressive anti-vote-fraud effort.
"The Justice Department really made prosecution of voter fraud of this sort a big priority in the first half of this decade, and they really didn't come up with anything," he said.
"We're chasing these ghosts of voter fraud, like chickens without a head," said Lorraine Minnite, a political science professor at Barnard College in New York who has researched voter fraud and fraud claims for most of the past decade. "I think it's completely overblown, I think it's meant to be a distraction."
"This stuff does not threaten the outcome of the election," said Minnite. "How many illegal ballots have been cast by people who are fraudulently registered to vote? By my count, it's zero. I just don't know of any, I've been looking for years for this stuff."
http://www.abcnews.go.com/Blotter/story?id=6049529&page=1Even the U.S. Department of Justice crime statistics cast doubt on the existence of widespread voter fraud. According to a report by the Justice Department's Criminal Division of prosecutions between October 2002 and September 2005, the Justice Department charged 95 people with "election fraud" and convicted 55. Among those, however, just 17 individuals were convicted for casting fraudulent ballots; cases against three other individuals were pending at the time of the report.
Further, on April 12, 2007, The New York Times reported, "Five years after the Bush administration began a crackdown on voter fraud, the Justice Department has turned up virtually no evidence of any organized effort to skew federal elections, according to court records and interviews."
http://www.nytimes.com/2007/04/12/washington/12fraud.html?_r=1&pagewanted=all&oref=slogin