Former US president Donald Trump has whipped thousands of fans into a frenzy at a rally on the eve of midterm elections – teasing when he will finally reveal if he plans to run again in 2024.

But Mr Trump, who spoke to a crowd of about 2000 at Dayton International Airport in Vandalia late on Monday (local time), stopped short of confirming what his supporters most want to hear.

“I’m going to be making a very big announcement on Tuesday, November 15, at Mar-a-Lago in Palm Beach, Florida,” he said towards the end of his speech.

At a visit just hours before voting opened, he urged his fans to back Senate candidate JD Vance. He said Mr Vance would be independent, and focused on Ohio’s interests – and “won’t be owned by me unfortunately”.

Mr Trump did not say what his announcement would be, but he has hinted for months about another run for president in 2024.

Monday night’s letdown followed multiple reports in the US media that he had been pondering using his Ohio visit to finally reveal his decision. He also offered his own hints.

“I ran twice, I won twice and I did much better the second time than the first,” Mr Trump said at a rally in Miami on Sunday (local time).

“In order to make our country successful, safe and glorious, I probably have to do it again. Stay tuned tomorrow night in the great state of Ohio. Stay tuned.”

In the end, he disappointed the crowd.

The Ohio speech followed a warning from President Joe Biden that a Republican victory in this week’s US midterm elections could weaken America’s democratic institutions and undo many of the accomplishments of his presidency.

“Today we face an inflection point,” Mr Biden earlier told his own cheering crowd of supporters at Bowie State University, a historically black college outside Washington.

“We know in our bones that our democracy’s at risk and we know that this is your moment to defend it.”

Mr Biden’s comments reflected the deep political divide in the US ahead of Tuesday’s elections that could deliver control of one of both chambers of the US Congress to the Republicans.

Non-partisan election forecasters predicted on Monday that Republicans were likely to pick up roughly 25 seats in the 435-seat House of Representatives, more than enough to win a majority.

Analysts said Republicans also could pick up the one seat they need to win control of the Senate.

Republicans have blamed Mr Biden’s administration for rising prices and crime, two top voter concerns.

But dozens of candidates also have echoed Mr Trump’s baseless claims of fraud in his 2020 election defeat. Some of them could end up as governors or election administrators in battleground states and play a central role in the 2024 presidential race.

Despite Mr Biden delivering on campaign promises to boost infrastructure and clean energy, many Americans have soured on his leadership.

Only 39 per cent approve of his job performance, according to a Reuters/Ipsos poll published on Monday.

Mr Biden’s unpopularity has made him an unwelcome guest in the most competitive races.

On Monday, he spoke in reliably Democratic Maryland, where the Democratic gubernatorial nominee, Wes Moore, is widely expected to win back the Republican-held governorship.

If Republicans win the House or the Senate, it would spell the end of Mr Biden’s efforts to get abortion protections, family leave benefits and other Democratic priorities through Congress.

It also would open the door for two years of Republican-led investigations that could potentially damage the White House.

A Republican-led Senate could also block Mr Biden’s nominations for judicial or administrative posts.

Billionaire Elon Musk, whose purchase of Twitter and vows to loosen the reins on who can say what on the platform has led to some speculation that it could unleash a wave of disinformation, tweeted on Monday that “independent-minded voters” should vote for a Republican Congress “because shared power curbs the worst excesses of both parties.”

If Republicans secure a House majority, they plan to use the federal debt ceiling as leverage to demand deep spending cuts.

They would also seek to make Mr Trump’s 2017 individual tax cuts permanent and protect corporate tax cuts that Democrats have unsuccessfully tried to reverse over the past two years.

Control of Capitol Hill would give Republicans the power to block aid to Ukraine. But they are more likely to slow or pare back the flow of weapons and economic assistance to Kyiv than stop it.

Trump supporters, spurred by his false election claims, have threatened and harassed election workers and voters.

The US Justice Department said it would monitor voting in 64 locations across the country.

But the White House said on Monday that law enforcement had not reported any specific, credible election-related threats.

More than 43 million Americans have already cast their ballots, either in person or through the mail, according to the US Elections Project, which tracks early voting.

Experts say it might be days or weeks before the outcome of some close races – and control of Congress – is clear.

-with AAP