Trump's midwestern stop in Iowa is part of a push through states where polling points to some tight races.
Trump held a rally in Sioux City as Republicans defend a Senate seat, three House districts and the governor's office, giving Democrats an opening after years of Republican gains.
"Pretend I'm on the ballot, because if you get out, we're going to win big," Trump told the crowd gathered in an airport hangar.
He has spoken repeatedly with confidence about his connection with voters, though his approval rating has fallen and some Republicans Trump is trying to help have broken with him over the Iran war, as higher fuel and grocery prices deepen voters' economic worries.
Republican representative Ashley Hinson, who also addressed the Sioux City rally, has called for an "immediate end" to the war, saying Iowans should not bear higher fuel or grocery prices because of it. She is in a close Senate race against Democratic state Representative Josh Turek.
"If we lose this seat here in Iowa, the Democrats will control the US Senate. Mark my words," she said.
Republican Representatives Mariannette Miller-Meeks and Zach Nunn, facing difficult reelection races, joined Democrats last month in voting to limit Trump's war powers in Iran.
"The only thing is the fuel prices that are hurting him more than anything," said rally attendee Ron Sitzmann, 73, a semi-retired parts and service director for an automotive company in Iowa, adding that he thought prices would drop once the conflict ended.
"I just wish it would happen sooner than later because it's kind of dragging on," said Sitzmann, who remains a strong Trump supporter.
Trump was in Iowa to back his party's candidates but used the platform to attack Democrats from other states, including Somali American Minnesota politician Ilhan Omar, suggesting that she should quit Congress in the wake of a raid by federal agents on her home.
Omar says the government is being weaponised to target her as a political enemy of Trump's, who has been ramping up anti-Somali and anti-Muslim rhetoric in recent weeks.
In the Iowa races, Democrat Rob Sand leads Republican Zach Lahn in recent polling for governor, while Democrats are targeting an open House seat in the state's second congressional district.
Republican losses would be a setback for Trump in a US state where his popularity helped the party build a commanding advantage.
The Senate and House races could help decide control of Congress, shaping his ability to advance his agenda during his final two years.
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