US President Joe Biden insisted on Friday that Democrats will win the midterm elections in the United States next week but warned of a challenging two years if polls predicting Republican victory are true.

Biden said he was optimistic that his Democratic Party could prevail, during a multi-state push in the final days before Tuesday's ballot. "Folks, I'm not buying the notion that we're in trouble," he told an audience in Chicago. "I think we're going to win. I really do."

Republicans are poised for potentially large gains, according to polls, in Tuesday's congressional election, expecting to gain control of both the House of Representatives and the Senate.

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This would transform the final two years of Biden's first term into a dogfight, making way for a contentious presidential contest in 2024, with questions about whether Biden, who turns 80 this month, will want to run again.

"If we lose the House and Senate it's going to be a horrible two years," he said. "The good news is that I'll have the veto pen," he added, referring to his power to block bills from becoming law.

Biden has returned to the campaign trail, as has his defeated 2020 opponent.

Former President Trump, who has spent much of the last two years wallowing in his defeat and promoting the debunked claim of election fraud, used a rally appearance to offer the strongest signal yet that he is considering a return campaign.

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During a rally Thursday in Iowa, the first state to hold its Republican nominating contest in presidential elections, Trump said, "In order to make our country successful and safe and glorious, I will very, very, very probably do it again, OK? Very, very, very probably", adding, "Get ready. That's all I'm telling you. Very soon. Get ready. Get ready."

According to a report in Axios on Friday, aides are firming up plans for a November 14 announcement.

However, how well the right-wing candidates supported by Trump perform on Tuesday will probably have an impact on that. The investigation into Trump's storage of top-secret information at his Florida golf estate and the possibility of an indictment on major criminal charges are other factors at play.

For now, Republicans are showing confidence that they can at minimum get the one state they need to move the Senate from 50-50 to their own narrow majority, while expecting solid gains in the House of 12 to 25 seats, overcoming the Democrats' current eight-member advantage.

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