Phoenix, Arizona - A man who repeatedly threatened to kill an Arizona election official was sentenced to two and a half years in prison on Monday, as investigators warned of increasing hostility towards poll workers ahead of this year's high-stakes election.

Joshua Russell, from Ohio, directed numerous expletive-laden tirades against the person chiefly responsible for the smooth running of the 2022 midterm election in Arizona, the state's department of justice said.

"Mr. Russell made three phone calls to the office of then-Secretary of State Katie Hobbs threatening to put her in the ground or in a grave," department chief Gary Restaino told reporters on the day Russell was jailed for two-and-a-half years.

Hobbs, who is now Arizona governor, has for years been the target of vitriol from people who falsely believe that Donald Trump won the 2020 election.

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His narrow loss in Arizona was one of several key upsets that saw Joe Biden swept into the White House, with the previously Republican-led state becoming something of a Ground Zero for election deniers.

Trump, who has once again clinched the Republican Party nomination, continues to insist without evidence that he won.

Russell's sentencing comes just weeks after a Massachusetts man was jailed for three and a half years for threatening to bomb the Arizona secretary of state's office in 2021.

Federal charges have also been filed against five other people who have made threats against officials in the state.

"Death threats are not debate," said John Keller, a member of a Justice Department task force dedicated to the security of US election officials.

"Death threats do not contribute to the marketplace of ideas. Death threats are not First Amendment protected speech."

"Death threats and any threats of violence are condemnable, criminal acts, and they will be met with the full force of the Department of Justice."

A 2022 survey showed intimidation of poll workers is alarmingly common in the US, with one in four telling the Democracy Fund foundation they had been threatened as Republicans continue to push false election fraud claims.