Thousands of Coloradans have been denied the right to vote because of a policy that may violate federal law.

Colorado Secretary of State Mike Coffman has authorized county clerks to purge newly registered voters under the so-called 20-day rule. Here, county clerks must send non-forwardable letters to newly registered voters. If the mail bounces back to the clerks, then they must remove the voter applicants’ names from the rolls.

Voting rights advocates say that the policy violates the 1965 National Voting Rights Act. The Advancement Project, a voter protection organization, filed suit on behalf of several other groups against Coffman to halt the practice and reverse course on other voter purges. According to the suit, 3,291 of these 20-day applicants have been removed since August 2007.

“We consider these voters to have been registered when they are entered into the SCORE database,” Jenny Flanagan, executive director of Colorado Common Cause told the Colorado Independent, referring to the state’s new voter database. “They have met the other requirements for registrations and they are taken off when the cards are returned. There are examples of when there could have been an error.”

Individuals ejected by the 20-day rule are among the 30,000 purged voters that make up the basis of the lawsuit. According to the complaint, Coffman also canceled duplicate registrations and registrations of people who moved. The NVRA specifies that the state may only cancel three types of voters within 90 days of a federal election: deceased people, felony convicts, and those who withdraw their own names.

Flanagan, who is also a plaintiff in the case, says that the 20-day rule is subject to human fallacy. A would-be voter might make a mistake on his or her own address on the form. But so could a registrar at the clerk’s office when entering the data. Postal employees aren’t immune to slip-ups either.

“We believe that it should not be allowed any time,” she says. “There are efforts to protect voters during this election, but there are some long term solutions we are seeking. We want to end this practice of canceling registrations and get the state in compliance with the NVRA.”

The NVRA does allow for voters to be removed from state rolls, but only after county clerks have sent a forwardable mail confirmation and waited two election periods to see whether the voters shows up to vote, according to Sarah Brannon, staff attorney with the Fair Elections Legal Network, a group acting as legal counsel on the lawsuit. “[Applicants] can’t be removed within 20 days,” she says.

Brannon says that the 20-day rule doesn’t disproportionately affect one population or another. But a Colorado Independent report published earlier this month showed that homeless people in particular are impacted by the legislation. Many homeless voters register using shelters or day centers as their mailing address. Counties send confirmation forms to these locations. But if homeless people don’t turn up within a week or two to pick up the mail, shelters typically return it to the sender. Which means that the homeless individuals are struck from the rolls.

Colorado isn’t the only state with a 20-day rule on the books. In Michigan, the American Civil Liberties Union and the Advancement Project — the same group bringing forward the suit in Colorado — successfully challenged a similar law; the state was ordered to stop purging voters whose confirmation cards were returned to county clerks.

The case will be heard on Wednesday afternoon.

Coffman’s office did not return repeated requests for comment. He has defended his purges in the past; Attorney General John Suthers has also backed Coffman, saying that the cancellations did not violate the NVRA.