Ted Cruz has hinted he will send Hillary Clinton to prison if he becomes the next US president.

The Republican candidate made the threat on Monday at a rally in Irvine, California, where he also declared that the state will decide the GOP nomination.

Cruz fired up the crowd with a vow to keep the Democratic frontrunner from the White House – and possibly behind bars. “I’ve got slightly different government housing in mind for her,” he said.

The Texas senator did not elaborate but the threat – the latest sign of intense polarisation – was a reference to the FBI’s investigation into Clinton’s use of a private email server while secretary of state.

Ted Cruz crowd snapping up jail Hillary badges. Last election I covered with candidate jail threats was Venezuela. pic.twitter.com/tboCu1yZlJ — Rory Carroll (@rorycarroll72) April 11, 2016

It was unclear how serious he meant it but the audience in the Irvine hotel roared approval and snapped up “Hillary for prison” badges after the event, citing the email server and her alleged role in the death of four Americans at a US outpost in Libya in 2012.

Cruz made a more obviously jokey reference last year to Clinton ending up in Leavenworth, a federal prison in Kansas.

Reviving the threat, or joke, played to the Republican base’s loathing for the former first lady and underlined Cruz’s attempt to position himself as the party’s best chance of beating her in November’s election.

Audience members assumed Cruz was serious. “If Cruz gets in I think he will appoint an attorney general to indict her. The server was patently illegal. It violated the Espionage Act,” said Robin Hall, 65, a retired education administrator.

“Apparently she broke the law, and it seems that under Obama the Justice Department is very politicised and doesn’t go after Democrats,” said Michelle Markel, who accompanied her husband, Bob, 67, who held a “We love Lion Ted” poster.

Christopher Underwood, 31, wearing a stars and stripes shirt and Duck Dynasty baseball cap, said Clinton deserved jail not just for the use of a private email server but criminal bungling. “She committed federal crimes. People died in Benghazi. She sent people to die.”

Michele and Bob Markel in Irvine for lion Ted. 'He's a biter. He's a fighter. He's not afraid.' #tedcruz pic.twitter.com/5DKyl0ofzj — Rory Carroll (@rorycarroll72) April 11, 2016

A vendor outside the hall ran out of “Hillary for prison” badges within minutes of the speech ending.

They were selling well even before Cruz took to the podium. “At first people think it says ‘Hillary for president’ and it’s ‘oh no’,” said Jake Towe, a Cruz supporter who travelled with his father from Oklahoma to sell this and other badges. “Then they look closer and it’s ‘oh, I love it’.”

The use of a private server for official business has dogged Clinton, though she remains favoured to beat Bernie Sanders and clinch the Democratic nomination. She has apologised for using the server while in office from 2009 to 2013 but denied wrongdoing. At the FBI’s request the state department has suspended plans for an internal review of whether classified information was properly handled.

Jack Pitney, a politics professor at Claremont McKenna College, said just about any anti-Clinton comment would get a positive reaction from a GOP audience. “Both parties show an increasing tendency to view the other side not just as mistaken, but fundamentally evil. As far back as 2007, 35% of Democrats thought that Bush knew about 9/11 in advance. And now many Republicans see Clinton as part of a criminal conspiracy.”

Taking time off from campaigning for New York’s 19 April primary to appear on the other side of the country in Orange county, followed by a scheduled stop later on Monday in San Diego, showed Cruz’s early focus on California’s 7 June primary.

He also visited in late March for a fundraiser and to appear on Jimmy Kimmel’s ABC late-night talkshow.

Donald Trump, the GOP frontrunner, cancelled an event near Los Angeles last week in order to focus on New York, reinforcing a perception that Cruz has a more forward-thinking strategy to rack up delegates before the party’s possibly contested convention in Cleveland in July.

California has 172 delegates, more than any other state, but coming so late in the primary calendar it traditionally has had no say in deciding the nominee.

Cruz hailed his hosts – hundreds of cheering fans in the Hotel Irvine – as kingmakers. “I’m going to say something that has not been said in 50 years: California is going to decide the nomination.”

He appealed to the crowd to join a volunteer army more than 265,000-strong, working their phones from home to lobby friends and neighbours. “The polls show that with me as nominee we beat Hillary Clinton.”

Most polls suggest Cruz would in fact lose to the Democrat but several crowd members said he was still the best bet. “Trump – he’s too radical. I don’t think he’s electable. If I did, I’d vote for him,” said Gary Howard, 71, who used to work in the grocery business.

His wife Judy, 70, said Cruz’s unpopularity with Senate colleagues gave her pause but that she believed he will secure the border. “It’s too loose. We’ll have more terrorists.”