Phoenix, Arizona - An election official in Arizona is to ask a court Tuesday to rule on whether almost 100,000 long-term residents will be able to vote in state and local polls this November after discovering they had not proved their US citizenship.

The ruling will not affect the ballot for the presidential race in Arizona, which Democrat Joe Biden won in 2020 by a wafer-thin 10,400 votes, but could add to uncertainty in one of the most hotly contested states in this year's knife-edge White House race.

It will also likely fuel conspiratorial claims by Donald Trump's allies who say, without evidence, that voter rolls are inaccurate and allow non-citizens to illegally participate in elections.

Arizona is the only state that requires voters to provide proof of citizenship to be able to vote in state and local elections. All others require only that they swear under penalty of law they are eligible.

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While Arizona's lawmakers originally intended the proof of citizenship requirement to apply to all elections, a 2013 US Supreme Court ruling said the provision could not apply to federal elections.

Stephen Richer, the Republican recorder of Maricopa County, the state's most populous county, said a review of records found a flaw in registration procedures meant the system would often inaccurately assume applicants who received a driving license before 1996 had already provided this proof.

"The number is about 97,000 registrants across the state," he wrote on X, the social media platform formerly known as Twitter.

"All of these people have attested under penalty of law that they are US citizens. And, in all likelihood, they almost all [are] US Citizens. But they have NOT provided documented proof of citizenship."

Richer said people who have not satisfied this requirement would still be allowed to vote in federal elections, including that for president, but under Arizona law, should not be given ballots for local polls, including a referendum that could formalize a right to abortion in the state.

"The [Arizona Secretary of States] argues that it is too close to the election to implement such a change and that it would be unduly burdensome on voters and deprive them of their voting rights.

"That is why we are going to the courts. To get a clear answer."

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Richer has a reputation as a fair-minded and non-partisan election official who has raised the ire of Trump's MAGA Republicans because of his refusal to bend to their demands.

Arizona has around 4 million registered voters.

After Biden's narrow 2020 win, it was the locus of Republican Party anger and unproven allegations of voter fraud, with repeated false claims that undocumented migrants had participated in the poll.

Extremist members of the party, including defeated gubernatorial candidate Kari Lake, alleged Democrats had connived to pad the rolls to their advantage and have been searching for ways to pare them back.

Ironically, Richer's lawsuit affects mostly older, long-term residents of the state rather than recent migrants, and, he says, the majority of them Republicans.

"Largest category is between ages 45-60," he wrote. "Partisan breakdown reflects state – plurality Republicans."