President Joe Biden made a final appeal on the eve of Tuesday's midterms for Democrats to protect democracy by defeating Donald Trump's Republicans in an election that polls show could upend power in Washington.

With Republicans well placed to win at least partial control of Congress and Trump hinting at a 2024 White House comeback bid at his own rally late Monday, Democrats braced for the worst.

Even if Republicans win only the House of Representatives, that would scupper Biden's legislative agenda for the last two years of his first term and potentially lead to a weakening of US support for Ukraine's resistance against Russia, reports AFP.

"Our lifetimes are going to be shaped by what happens," Biden told an enthusiastic crowd at a historically Black university in Bowie, near Baltimore, late Monday. "We know in our bones that our democracy is at risk and we know that this is your moment to defend it."

"The power's in your hands," he told Democrats. "So vote, get out the vote."

An influx of far-right Trump backers in Congress would also accelerate the shift that has been taking place inside the Republican Party ever since the former real estate tycoon stunned the world by defeating Hillary Clinton for the presidency in 2016.

Despite facing criminal probes over taking top secret documents from the White House and trying to overturn the 2020 election, Trump is now using the midterms to cement his status as the de facto Republican leader and presumptive presidential nominee.

In a typically dark, rambling speech to fans in Dayton, Ohio, Trump said, "if you support the decline and fall of America, then you must, you absolutely must vote for the radical left, crazy people."

"If you want to stop the destruction of our country, then tomorrow you must vote Republican in a giant red wave."

- Lame duck? -

More than 40 million ballots had been cast through early voting, meaning the outcome was already taking shape before polls opened nationwide Tuesday.

Biden tried to remain upbeat during his closing address in Baltimore, but in a call with party allies earlier Monday he conceded that his dreams of keeping Congress, which Democrats currently control by a thin margin, amounted to a "very high expectation."

Trump Republicans are "some of the darkest forces we've ever seen in our history," he said.

Biden's speech laid out what he said was "a choice between two very different visions of America," arguing that his administration has successfully steered the world's largest economy out of the Covid pandemic, with unemployment at 3.75 percent and manufacturing industries on the rise.

Republicans, he said, would return to "trickle-down economics" that favor the rich.

However, polls show that Republican messaging emphasizing four-decades-high inflation, crime and illegal immigration has left voters in an angry mood. As the party controlling the White House and -- albeit barely -- Congress, the Democrats are likely to get punished.

If Democrats can't even hold the Senate, then Biden would find himself in a state of constant political warfare in Washington. There would also be immediate, harsh questions over whether Biden, who turns 80 this month, should seek a second term or give way to a younger party member.

Kevin McCarthy, who would likely become Republican speaker of the House -- placing him second in line to the president -- refused to rule out impeachment proceedings.

"We will never use impeachment for political purposes," McCarthy told CNN. "That doesn't mean if something rises to the occasion, it would not be used at any other time."

Biden's entire agenda would go into deep freeze. That would raise questions over everything from climate crisis policies, which the president will be laying out at the COP27 conference in Egypt this week, to Ukraine, where Republicans are reluctant to maintain the current rate of US financial and military support.

While insisting he supports Ukraine's struggle, McCarthy told CNN there could be no "blank check" -- a nod to the isolationist Trump wing of his party and a signal likely sending shivers through Kyiv.

Adding to tensions -- and a reminder of Moscow's murky role throughout Trump-era US politics -- Kremlin-connected oligarch Yevgeny Prigozhin boasted that Russia was trying to tilt the outcome.