The move in Pennsylvania came as analysts suggested polls from right-leaning groups might be exaggerating his popularity - and would be used by Trump to claim cheating prevented him from returning to the White House.
The warnings - from Democrats and anti-Trump Republicans - come as Americans prepare to vote on Tuesday in the most consequential presidential contest in generations.
Most polls show Trump running neck and neck with Kamala Harris, the vice-president and Democratic nominee, with the two candidates seemingly evenly matched in seven swing states.
They both held rallies in Las Vegas, in the battleground state of Nevada, on Thursday night, with Harris appearing alongside Jennifer Lopez and telling the crowd: "If he were re-elected, Donald Trump would walk into that office with an enemies list. When I am elected, I will walk in with a to-do list."
Trump told his supporters: "She is the worst vice-president in the history of our country. Kamala, you're fired! Get the hell out of here. She is horrible.
And she lies. Every time she gets up she tells a lie." But suspicions have been voiced over a spate of recent polls, mostly commissioned in battleground states from groups with Republican links, that mainly show Trump leading. The projection of surging Trump support as election day nears has drawn confident predictions from him and his supporters.
"We're leading big in the polls, all of the polls," Trump told a rally in New Mexico on Thursday. "I can't believe it's a close race," he told another rally in North Carolina, a swing state where polls show he and Harris are in a virtual dead heat.
Trump, who falsely claims that Joe Biden stole the 2020 election, is also paving the way for repeating the accusation via legal means.
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