Trump has embarked on a campaign blitz ahead of the November 3 mid-term elections, travelling around 50,000km since late September to boost Republican candidates.
His second stop in North Carolina in as many months was a signal of how much is at stake in this southern state.
Polls show former Democratic Governor Roy Cooper with a commanding lead over Republican Michael Whatley, a former senior party official, in a Senate race that could help decide the balance of power in Washington.
The two are running to fill the seat of retiring Republican Thom Tillis, a one-time Trump ally, who split with the president over his landmark tax bill and defence policy.
Tillis told Politico that his fellow Republicans were headed toward "a train wreck" and a "whoop ass" in November.
But Trump, speaking in the city of Greenville on Tuesday night, predicted Whatley would prove the doubters wrong.
"I think you're gonna see some amazing results," he told hundreds of supporters who gathered on the grounds of the local airport.
"They never poll us right."
Trump won North Carolina in each of his three presidential runs, but Democrats are banking on a breakthrough performance this year, with polls showing a clear majority of Americans dissatisfied with rising prices and the war in Iran.
The president's approval rating has fallen to 31 per cent, since he ordered military strikes on Iran in February alongside US ally Israel, triggering a surge in gasoline and diesel prices.
Some 68 per cent of Americans disapprove of Trump's handling of the economy, the latest Reuters/Ipsos found, including 37 per cent of Republicans.
Towards the end of his speech, the president repeated his promise to give $US5,000 ($A7,125) to every US adult should Republicans maintain their hold on Congress.
"If we lose," he said, "you get nothing".
