Republicans argue that voting fraud is no small affair, even if the cases are few, when some elections are decided by hundreds of votes.
The different viewpoints underscore a divide between the parties: Are the small numbers of voting fraud evidence that a problem exists? Or do they show that the voter registration system works?
Colorado: Last year, Secretary of State Scott Gessler, a Republican elected in 2010, estimated that 11,805 noncitizens were on the rolls. But the number kept getting smaller.
After his office sent letters to 3,903 registered voters questioning their status, the number of noncitizens now stands at 141, based on checks using a federal immigration database. Of those 141, Gessler said 35 have voted in the past.
Florida: The search began after the state's Division of Elections said that as many as 180,000 registered voters weren't citizens. Like Colorado and other states, Florida relied on driver's license data showing that people on the rolls at one point showed proof of non-citizenship, such as a green card.
Florida eventually narrowed its list of suspected noncitizens to 2,600 and found that 207 of them weren't citizens, based on its use of the federal database called SAVE, or the Systematic Alien Verification for Entitlements. The system tracks who is a legal resident eligible to receive government benefits.
Of the 2,600 initially marked as possible noncitizens, about 38 percent were unaffiliated voters and 40 percent were Democrats, according to an analysis by the Miami Herald. The state has more than 11.4 million registered voters, so the 207 amounts to 0.001 percent of the voter roll.
North Carolina: The nonpartisan state elections board last year sent letters to 637 suspected noncitizens after checking driver's license data. Of those, 223 showed proof they were citizens, and 79 acknowledged they were not citizens and were removed from the rolls along with 331 who did not respond to repeated letters, said Veronica Degraffenreid, an elections liaison for the board.
She said the board did not find evidence of widespread fraud, noting there were only 12 instances in which a noncitizen had voted. North Carolina has 6.4 million voters.
Michigan: Secretary of State Ruth Johnson, a Republican, last week estimated that as many as 4,000 noncitizens are on the state's voter roll.
The department said it verified 1,000 registered voters who are noncitizens, based on an analysis of about 20 percent of complete citizenship data. She extrapolated the 4,000 number from a census survey, which showed Michigan has a noncitizen population of 304,000.
Ohio and Iowa also are negotiating to use the SAVE database to verify citizenship, although it's unlikely they will have enough time to do anything before the Nov. 6 election.