The US Department of Justice has filed criminal charges against 28 green card holders in California accused of fraudulently registering to vote and casting illegal ballots.

The crackdown marks another escalation in the federal government’s ongoing efforts against alleged noncitizen voting, efforts that Democrats and voting rights groups say are part of a strategy by the Trump administration to sow doubt in the results of the upcoming midterm elections.

The justice department announced Monday that 28 noncitizens from across California were facing charges, including making false statements in a naturalization application and fraudulently signing up to vote, felonies that could carry five years in prison. The defendants are lawful permanent residents, who have legal status in the US but are not citizens authorized to vote.

A justice department spokesperson said 21 of the 28 defendants had cast unlawful ballots. Eleven of the defendants have already been taken into custody and arrests were ongoing, the justice department said. Prosecutors alleged the defendants lied under oath in voter registration applications.

Voting by noncitizens is extremely rare, and election experts have raised concerns that the justice department’s crackdowns could disenfranchise US citizens.

The arrests came days after the justice department sued California, demanding the courts block the state from tabulating certain ballots in the high-stakes November midterms. The justice department had previously sent Los Angeles county election officials a thumb drive that purportedly contained the names of noncitizens who were unlawfully registered to vote. The justice department demanded that California officials, within 48 hours, confirm whether those on the list were citizens and set aside the ballots of those who remained unconfirmed.

The justice department’s lawsuit said it had sent California officials a list of more than 20,000 people who the federal government “believes are likely noncitizens” who were sent ballots by mail.

California’s secretary of state, Shirley Weber, has condemned the justice department’s demands as “outrageous”, saying federal officials had for months made “vague and unsubstantiated allegations” about alleged voter fraud in the state. In a letter last week to the justice department, Weber, a Democrat, noted that the Department of Homeland Security (DHS) first claimed in July that more than 190,000 registered voters in the state “may be” noncitizens, but had since revised that number down to 20,000 while continuing to refuse to share details about its data and methodology.

Weber said that “treating the thousands of voters on DOJ’s list as ineligible until proven otherwise” would violate federal law that prohibits states from systematically removing the names of ineligible voters from voter lists in the lead-up to a federal election.

There are 23.1 million registered voters in California, according to recent state figures.

Trent McCotter, the acting US deputy attorney general, acknowledged the government’s list of alleged noncitizen voters in California might not be accurate when pressed by a reporter to provide more details about the methodology.

“There may be some names on there who, it turns out, ultimately are citizens. We’re not perfect, and that’s why we’ve asked California to cooperate with us,” McCotter said.

On the government’s changing estimates about the volume of allegedly fraudulent registrants in the state, McCotter said: “Think of it like 190,000 is the ceiling, 20,000 is the floor. Twenty-thousand are the ones we’re the most confident about … 190,000 includes ‘medium confidence’, ‘lower confidence’ – kind of the initial cut that have not been manually verified yet.”

McCotter sent a letter to election officials on Friday threatening to prosecute any official that tabulated a ballot from a noncitizen. The threat is legally specious, experts say, and a blatant effort to intimidate officials.

Some federal efforts to root out alleged noncitizen voters have faltered in other states. Last month, Nevada state officials said DHS had wrongly identified 185 people on the state’s voter rolls as noncitizens.

The justice department said Monday that in recent weeks, federal prosecutors have filed charges against a total of 112 people across 20 states for alleged registration offenses and illegal voting.