PHOENIX, Nov 8, 2022 (BSS/AFP) - Election officials in a hotly contested part of Arizona said security measures had been beefed up for Tuesday's midterm elections, and warned they had "zero tolerance" for voter intimidation.

Millions of Americans are expected to head to the polls to elect national, state and county officials, in one of the most eagerly-watched ballots of recent years.

In Maricopa County, which became ground zero of election denialism after its votes pushed Joe Biden ahead of Donald Trump in 2020, authorities said they were doubling down on poll protections.

"We will be zero tolerance," Sheriff Paul Penzone told reporters.

"If you choose to come out there and break the law, I have plenty of cells for you."

The county of 4.5 million people has seen increasing tension since the 2020 election, with mistrust over voting systems a badge of honor among many in the state's Republican Party.

The party's candidates for secretary of state, governor and US senator all subscribe wholly to the denialism that Trump has pushed since losing the election.

Numerous reviews of the ballot have failed to find any evidence of fraud.

But that has not prevented masked poll watchers, some of them armed, from hanging around vote drop boxes, in what they said was an effort to prevent ballot stuffing.

Last week a judge ordered them to keep their distance and said they could not display their weapons anywhere near the boxes.

Penzone said he would deploy plainclothes officers to guard the voting centers, and protect the transfer of ballot boxes.

"I never thought in my life that we'd be putting up physical barriers, fence lines, barbed wire... to protect a building where... the free vote takes place," he said.

"But we are who we see in the mirror right now. And that is a result of... misinformation."

Voters in Phoenix said the denialism that has stalked Arizona since 2020 is worrying.

"It's a very tense climate right now," said 51-year-old Shana Ellis.

"Everyone should be afforded the right to vote and shouldn't be nervous about going to cast a ballot."

Officials said they were expecting a large turnout on Tuesday at the more than 200 polling places they had set up.