Donald Trump on Monday announced what he called the largest steel plant in American history to be built in Iowa, giving Republicans a jobs message to take to voters in a state that could help decide control of the Senate in November.

“Using advanced steelmaking technology, the plant will produce some of the highest quality, most affordable steel anywhere in the world,” Trump said in the Oval Office, flanked by Iowa’s top Republicans.

The plant, which “they’ve already started building”, Trump said, will be built in eastern Iowa by the foreign-owned steelmaker Mesabi Metallics and is expected to cost about $15bn, according to a White House official. It aims to begin production in 2030, years after voters go to the polls in November.

In his announcement, Trump singled out Iowa’s Ashley Hinson, the Republican congresswoman who was in the Oval Office and who is running for the Senate seat being vacated by Joni Ernst, also in the room. “I saw some good polls just recently,” he told her. “Looks like you’re doing well.”

But the polls for that race are less rosy than Trump suggested. A Marist survey published last week found Hinson trailing Democrat Josh Turek by eight points in the conservative state, while an earlier Emerson College poll had Hinson narrowly ahead but found voters worried about tariffs. Democrats see the seat as one of four they need to retake the Senate.

Trump said the plant would run on iron ore from the company’s mine in Minnesota, which accounted for roughly 60% of iron ore production in the US in the 20th century. The president took the chance to attack that state’s Democratic governor, Tim Walz.

“They have one of the worst governors in history, a real stupid guy, but they do have good iron ore, which is more important, I guess,” Trump said. The two men have clashed over mining in the state, most recently over Walz’s August order halting permitting for mining projects near the Boundary Waters wilderness.

The Mesabi Metallics company itself, however, is owned by the India-based Essar Group, and US taxpayers are helping to pay for the expansion: the Export-Import Bank, a federal agency, will provide up to $10bn in financing.

When asked whether global outsourcing would be needed to build the plant, the chairman of Mesabi Metallics, Rewant Ruia, said “most of the critical materials” would come from the US but added “there are some materials that might be used that need to be imported for making and refining the highest quality steels”.

Still, the president credited his tariffs – the taxes the US charges consumers on imported goods – for the investment. “Everyone’s building their plant here because they don’t want to pay tariffs,” he said. “It’s really not that complicated. A lot of people say they don’t understand it, but they do. They just don’t like to give us credit.”

Trump said the project was “expected to create up to 6,000 new construction jobs, nearly 2,000 manufacturing and mining jobs, and add another $95bn to the US economy”.