In a victory for Republicans, the federal government has agreed to let Florida use a law enforcement database to challenge people's right to vote if they are suspected of not being US citizens.

The agreement, made in a letter to Florida governor Rick Scott's administration, grants the state access to a list of resident noncitizens maintained by the homeland security department. The Obama administration had denied Florida's request for months but relented after a judge ruled in the state's favor in a related voter-purge matter.

Voting rights groups, while acknowledging that noncitizens have no right to vote, have expressed alarm about using such data for a purpose not originally intended: purging voter lists of ineligible people. They also say voter purges less than four months before a presidential election might leave insufficient time to correct mistakes stemming from faulty data or other problems.

Democrats say that the government's concession is less troubling than some GOP-controlled states' push to require voters to show photo identification.

But Republicans count it as a victory nonetheless in their broad-based fight over voter eligibility, an issue that could play a big role in the White House race. That's especially true in pivotal states such as Florida, Colorado, Nevada and North Carolina.

Republican officials in several states say they are trying to combat voter fraud. Democrats, however, note that proven cases of voter fraud are rare. They accuse Republicans of cynical efforts to suppress voting by people in lower socio-economic groups who tend to vote Democratic.

The homeland security decision may affect places beyond Florida, because Colorado and other states have asked for similar access to the federal database.

After a judge recently ruled against federal efforts to stop Florida's aggressive voter-list review, Homeland Security agreed to work on details for how the state can access the federal Save database – systematic alien verification for entitlements – to challenge registered voters suspected of being noncitizens.

Florida has agreed that it can challenge voters only if the state provides a "unique identifier," such as an "alien number," for each person in question. Alien numbers generally are assigned to foreigners living in the country legally, often with visas or other permits such as green cards.

Unless they become naturalized citizens, however, they cannot vote.

The agreement will prevent Florida from using only a name and birthdate to seek federal data about a suspected noncitizen on voter rolls.

The Save list is unlikely to catch illegal immigrants in any state who might have managed to register to vote because such people typically would not have an alien number.

Scott, whose administration had sued Homeland Security for access to the Save list, said the agreement "marks a significant victory for Florida and for the integrity of our election system".

"Access to the Save database will ensure that noncitizens do not vote in future Florida elections," Scott said Saturday.

In a letter Monday, the department told Florida it was ready to work out details for providing access to the Save list. The letter was signed by Alejandro Mayorkas, director of US citizenship and immigration services.

It follows a flurry of legal actions between Florida and the federal government. On June 11, the justice department said it would sue Scott's administration on grounds that the state's voter-purge efforts violated voting rights laws.

The same day, Scott announced a lawsuit against homeland security seeking access to the Save list. He said it could be a valuable tool in determining who is a citizen. Two weeks later, a US judge blocked the federal attempts to stop Florida's voter review efforts; the Mayorkas letter soon followed.

A homeland security spokesman said the agency had no further comment.

Department officials told the Orlando Sentinel last month they had concerns about using the Save list for voter-review purposes. They said the list's information is incomplete and does not provide comprehensive data on all eligible voters, the newspaper reported.

Scott's administration hopes to restart a suspended voter registration purge that was hampered this year by faulty data and bad publicity. The review, using driver's license information, initially produced 180,000 voters' names considered worthy of checking. County election supervisors examined 2,625 people on the list. But more than 500 were soon found to be citizens, and the review was halted.

State records show that 86 noncitizens were removed from the voter rolls since April 11, and more than half of them had voted in previous elections.

Florida secretary of state Ken Detzner asked election officials Saturday to restart the review. He said it will "include a carefully calibrated matching process" between the state's driver and voter data "before any records are verified through Save."

While some noncitizens who are legal residents may knowingly try to register and vote, others apparently do so unwittingly. After obtaining a driver's license, some assume they also can vote, officials say.

Access to the federal Save list may catch such ineligible voters in Florida. They presumably would have an alien number and be listed in state motor vehicle records.

Voter-rights groups expressed concerns about Florida's efforts.

"No matter what database Florida has access to, purging voters from the rolls using faulty criteria on the eve of an election could prevent thousands of eligible voters from exercising their rights," said Jonathan Brater, a lawyer with the Brennan Center for Justice at the New York University law school. "Florida must use a more transparent and accurate process and must leave enough time for voters targeted for removal to be notified and correct errors," he said.

Some state governments have sought access to the federal database for years. Federal officials told Washington state in 2005 they saw no way to compare voters and the homeland security information.

Colorado has sought the federal data for a year. Colorado, which has a Democratic governor but a Republican secretary of state, Scott Gessler, has identified about 5,000 registered voters that it wants to check against the federal information.

Officials in the politically competitive states of Ohio, Michigan, New Mexico and Iowa – all led by GOP governors – are backing his efforts.

Gessler said 430 registered voters have acknowledged being ineligible, but an "unenforceable honor system does not build confidence in our elections".

Although Republican activists have repeatedly said fraud is so widespread that it has corrupted the political process and, possibly, cost the party election victories, about 120 people have been charged and 86 convicted as of last year.

In 2007, five years after the George W Bush administration launched a crackdown on voter fraud, the justice department found virtually no evidence of organized efforts to influence federal elections with ineligible voters.