Trump, who spent Saturday at his Mar-a-Lago club in Florida after weeks of campaigning across multiple states, won Iowa by double digits in 2024.
But polls show Democrats tied with or leading Republicans in many of the state's races, and Trump's approval among rural residents fell to a new low of 45 per cent in October, according to Reuters/Ipsos polling.
Kansas, which last elected a Democratic senator in 1932, is also facing an unexpectedly close race.
Trump's administration has made $US15 billion ($A21 billion) worth of promises to agriculture producers in the last few weeks, initiatives intended to help offset higher costs for diesel and fertiliser that farmers have endured.
The US Department of Agriculture separately plans $US13.8 billion in safety net program payments for eligible producers for the 2025 crop year, offsetting shortfalls for farmers who grow key crops such as corn, soybeans, wheat and rice.
On Thursday, the agency said it would purchase up to $US472.5 million in agricultural commodities to support producers and give to charitable programs.
Republicans initially cancelled the Iowa and Kansas stops only for them to be abruptly rescheduled as Trump works to preserve a Senate majority at risk of slipping away.
The travel is a reminder of how starkly the political environment has changed since Trump's last campaign.
In 2024, he worked to flip states that had previously backed Democrat Joe Biden in 2020.
This year, Trump has focused largely on states he has dominated before as voters, including previous supporters, question the economic fallout of his decision to impose tariffs and launch the war in Iran.
Thirty-five Senate seats and all 435 House seats are on the ballot on Tuesday.
Republicans hold a 53-47 Senate majority and a narrow edge in the House.
Losing either chamber would let Democrats block Trump's agenda for his final two years in office and launch investigations of an administration that has so far faced largely compliant Republican majorities.
More than 52 million people have already voted in person or by mail, according to the University of Florida's Election Lab, a record for a midterm and up from nearly 46 million who voted before election day in 2022.
Michigan Democrat candidate Abdul El-Sayed's former opponent in the senate primary, Haley Stevens, stunned party officials on Sunday by endorsing Republican Mike Rogers after Democratic tensions boiled over.
The Democrat Stevens endorsed Rogers after attending a Detroit church service with him.
The remarkable show of support for Trump's backed candidate in the tightly contested race comes after she was uninvited to a joint event the same day with El-Sayed, who defeated the congresswoman by a slim margin in the August primary.
"Our Democratic nominee for Senate said I was no longer welcome in his campaign and that I should stay away," Stevens said in a video announcing the endorsement.
"And he's right. And that's why I am endorsing Mike Rogers for the United States Senate today."
While Stevens has been critical of El-Sayed's comments surrounding Israel, she had recently reaffirmed she would vote for the former public health official who would be the first US Muslim senator if elected on Tuesday.
with AP
