Former President Donald Trump campaigned in southeast Iowa in the middle of a fall campaign push aimed at locking in supporters with large organising events.

As he has with his other recent travels to the leadoff caucus state, Trump stumped in an area that formerly supported Democrats but has embraced him, as the influence trade unions once held has shrunk and lost voters to Republicans.

Trump headlined an afternoon event in Ottumwa, where 2,500 packed the inside of an event hall at the Bridge View Center in Ottumwa. The small city is a hub in eastern Iowa and the seat of Wapello County, one of 31 counties Trump carried in 2016 that Democrat Barack Obama had won four years earlier.

It was Trump's second trip in two weeks to eastern Iowa, where he was drawing large crowds, as his campaign has sought to step up their press to urge voters to commit to supporting him in the January 15 caucuses, where more than a half-dozen other Republicans are vying to rise as a threat to his popularity within the party.

“With your support on Monday, January 15, we're going to win the caucuses in a historic landslide," Trump told the packed event hall in Ottumwa Sunday.

The crowd spent hours waiting outdoors to enter the Trump event, despite the sweltering heat uncommon for early October. It was a stark contrast to the roughly 200 who attended a campaign stop in suburban Des Moines on Saturday by former United Nations Ambassador Nikki Haley, who has gained renewed interest in Iowa since the first two Republican presidential debates.

Trump is expected back in the Waterloo and Cedar Rapids areas next week.

Trump, the first Republican to capture the county since the Eisenhower administration, campaigned the week before in northeast Iowa.

There, he drew about 1,400 to rural Jackson County along the Mississippi River and almost 2,000 to Dubuque County to the north. Like Wapello, Dubuque County had been a Democratic stronghold for decades before 2016.

Though aides said they were not specifically targeting counties that Trump flipped in 2016, they noted that he has had success in the eastern part of Iowa where manufacturing has declined sharply in the past two decades. His administration's renegotiation of the US trade pact with Canada and Mexico remains popular.

Rick Anderson and his wife Nancy, who were filing into the hall, are the kind of voters whom Trump's campaign would like to persuade to caucus for the candidate on January 15. They used to vote Democratic but switched in 2016 to support Trump. They have not attended Iowa's Republican precinct caucuses in the past.

Rick Anderson, a retired union millwright who co-owns a small business with his wife, is among the many long-time union members who kept Wapello County and others in Iowa's once-robust, eastern manufacturing corridor reliably Democratic-performing until Trump.

“We like what he says. He says Drill, baby, drill,' and that's got my heart. Because that's what's wrong with the country's energy. Solve that problem and you solve so many other problems," Anderson said. “Democrats have lost touch with people like us.” Trump later travelled with his motorcade 40 miles northeast to Leighton, Iowa, where he visited a soybean farm. There, he autographed a John Deere combine and touted his administration's trade policy as a boon for farmers, even as Mexico and Canada seek to renegotiate the 2020 United States-Mexico-Canada Agreement.

In front of the polished, green farm implement and a backdrop of amber soybeans, Trump also commented on the stop-gap government spending measure aimed at keeping the federal government from a partial shutdown. Trump said House Republicans, who hold a thin majority, “got very little.” “I think they could have done a much better deal,” he said.

As Trump maintains a strong lead in Iowa, his Republican rivals are scrambling for backing, hoping a strong showing can help them consolidate the non-Trump support.

Trump volunteers at the site held clipboards stacked with pledge cards and asked attendees whether they would commit to supporting Trump at the caucuses.

Trump arrived in Iowa after a two-day trip to California, where he picked up 6 million of his 74 million votes in 2020 while losing the state by 30 percentage points to Democrat Joe Biden.

In a fiery speech that delighted Republicans dejected after decades of Democratic control, Trump escalated his long-standing tough-on-crime message with calls for violent retribution against criminals.

People caught robbing stores should be shot, Trump said to applause. He raised money during his trip to Orange County, once a bastion of conservatism in Southern California that has become increasingly competitive.

While Trump's would-be Republican challengers sparred in the second primary debate earlier in the week, Trump was in another key blue-collar county in the general election battleground of Michigan.

Trump spoke during Wednesday night's debate in Macomb County, Michigan, north of Detroit at a non-union manufacturing plant, where he blasted Biden's push for electric cars amid an autoworkers' strike. Trump carried Macomb County twice after Obama did in 2008 and 2012.