A Reuters/Ipsos poll completed on Monday found it was the party of choice for 46 per cent of registered voters, compared with 38 per cent who picked Republicans.
All 435 seats in the US House of Representatives and 35 of the 100 US Senate seats will be contested on Tuesday.
Democrats are widely favoured to retake the House but face a greater challenge in the Senate.
Loss of control in either chamber would constrain Trump and empower Democrats to investigate his administration, while a Senate majority would also allow them to block his appointees to federal courts.
But with control of Congress at stake for his final two years in office, Trump devoted his last major speech before voting day to tout his plans for beautifying Washington DC and talk up trouble-plagued renovations at the Lincoln Memorial Reflecting Pool.
Overall, Democrats expressed overwhelming optimism about their party's chances, thanks largely to Trump's struggle to control rising prices and the war in Iran, which has already spanned eight months.
The US military conducted new defensive strikes on some Iranian targets in the Strait of Hormuz, even though Trump previously said he would not launch strikes on Iran before the midterms.
Trump acknowledged the election may not go well for him or his party.
"For some reason, the sitting president doesn't win the midterms, but we gave this a very strong shot," he said at the historic monument, where he talked extensively about new grass, rusty pipes and algae.
"We have a lot of great things planned for Washington ... but for that you don't have enough time."
He also mentioned his new Marine One helipad, a golf course and plans for a triumphal arch that are facing a court challenge.
Trump later urged Republicans to pretend he is on the ballot in an effort to increase voter participation.
"We have to vote. If our people vote, we win in a landslide," he told Fox News.
Tensions were running high in Michigan, where Haley Stevens, who lost the Democratic Senate primary to Abdul El-Sayed in a bitter contest, switched her backing to Republican Mike Rogers over the weekend.
El-Sayed - a physician and former public health official - largely ignored the controversy at multiple events.
Asked what he would tell nervous supporters, he had a simple message: "Go vote."
California was also in the spotlight after the Supreme Court swiftly quashed an attempt by the Justice Department to intervene and block vote counting mail-in ballots from more than 20,000 registered voters who federal officials alleged were likely non-US citizens.
California had argued the federal lawsuit would disenfranchise those voters based on vague accusations and that the process for identifying non-citizens was error-prone.
At the same time, a federal judge tossed out a criminal case accusing two ex-senior FBI officials of lying about their involvement in an investigation involving former President Joe Biden's son Hunter and material from his personal laptop.
US Magistrate Judge Zia Faruqui lashed the Justice Department for bringing what he said was a political prosecution before the elections.
The decision dealt another blow to the department's attempts to investigate people Trump viewed as adversaries and came after a federal court hearing last week delved into how the 80-year-old had exerted control over the department during his second term.
The White House said the former real-estate developer had held events in 20 states over the past 32 days.
Meanwhile, US senator Tammy Duckworth, the top Democrat on a subcommittee that oversees aviation regulation, asked government agencies for a full account of why they allowed a flyover of the White House by Air Force One last week as part of a children's Halloween party.
Attended by Trump, it forced a temporary halt to traffic at Reagan Washington National Airport that delayed flights, while city lawmakers voiced concerns that residents, officials and the airport received only 15 minutes' notice.
Duckworth said was a "dangerous stunt".
with AP
