Republicans and Democrats traded final blows Monday ahead of midterm elections that could upend Joe Biden's presidency, weaken Western support for Ukraine, and even open the door to a comeback bid by Donald Trump.

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

Biden acknowledged that Democrats face an uphill battle to retain control of Congress, telling supporters that "if we're able to hold on, we're going to be in an incredible shape."

"I know that sounds like a very high expectation," he admitted, adding that "we're up against some of the darkest forces we've ever seen in our history."

Biden, who has framed his closing argument as a warning that American democracy is on the line, was set to close out days of campaigning for Democratic candidates at a rally Monday evening near Baltimore.

Lame duck?

Trump — using the midterms to repeatedly tease a possible 2024 White House run, even as he faces criminal probes over taking secret documents and trying to overturn the 2020 election — was holding a rally in Ohio.

With polls showing Republicans in line to seize the House of Representatives, the increasingly far-right party eyed snarling the rest of Biden's first term in aggressive investigations and opposition to spending plans.

Kevin McCarthy, who would likely become 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."

One key question remained whether the US Senate would also flip, leaving Biden as little more than a lame duck.

With Congress out of Democrats' hands, Biden would see his legislative agenda collapse.