Democrats are facing a gargantuan struggle to hang on to Congress against the Republicans who look comfortably placed to snatch a House majority in key U.S. midterms Tuesday.

Joe Biden and Donald Trump headlined a frantic last day of campaigning Monday on the eve of the elections that will shape the rest of the U.S. president's term – and could pave the way for a White House comeback by his predecessor.

The midterm campaigns, cast as a "defining" moment for U.S. democracy by Biden, have largely been dominated by kitchen-table issues like inflation.

The Republicans look well-placed for a House majority on Tuesday, and many Democrats fear the Senate also slipping away in a defeat that would see Biden's foes in near total charge of legislation during his last two years in the White House.

Polls show most Americans are anxious about the economy and feel the country is on the wrong track, emboldening Republican candidates in districts that once looked out of reach.

With all 435 seats in the House of Representatives up for grabs alongside a third of the 100-member Senate and a slew of state posts, Democrats were putting a brave face on their prospects.

"The party in the White House usually loses during midterms but the reality is we still have a very strong pathway, not just to keeping the Senate but really picking up seats," New Jersey's Cory Booker told ABC on Sunday.

Democratic candidates have been lent star power on the campaign trail by the party's most popular elder statesmen, including previous presidents Barack Obama and Bill Clinton.

Republicans have tapped a narrower roster of their own political big hitters, with the campaign spotlight turning in recent weeks to Trump – who has been teasing a probable new run for the presidency in 2024.

Biden and Trump will go head-to-head on election eve, the president with a rally near the capital in Maryland, while Trump will be campaigning in a turbulent Senate race in Ohio.

'Wake-up call'

The political landscape has been tilting away from Democrats since the summer, with polls showing Republicans odds-on for a double-digit majority in the House.

"This is going to be a wake-up call to President Biden," was the bullish weekend prediction of Glenn Youngkin, the Republican governor of Virginia.

The Senate is more of a toss-up but Democratic hopes of keeping the upper chamber, which they control thanks to tiebreaking votes from Vice President Kamala Harris, hang in the balance.

Races in Pennsylvania, Nevada, Wisconsin, Georgia, New Hampshire and Ohio have narrowed to projected photo finishes, and any one of them could swing the balance of power.

Democrats have focused their closing arguments on voting rights, protecting abortion access and welfare – and in Biden's case, on the threat posed by growing support among Trump's Republicans for political conspiracy theories.

The Republicans counter that a vote for Democrats means no end to soaraway inflation and rising violent crime, seeking to make the midterms a referendum on the president.

With his approval rating marooned around 42%, Biden has largely avoided the most contentious states.

But he rallied alongside his former boss Obama in Pennsylvania Saturday, as part of a hectic agenda of late stump stops that has also taken him to Illinois, Florida and New York.

The president rebuked extremist supporters of "defeated president" Trump, telling the crowd: "Your right to choose is on the ballot. Your right to vote is on the ballot."

'Decline and fall'

Staging a rival weekend rally in the swing state, Trump – who continues to push false claims the 2020 election was stolen – accused the "radical, crazy" Democrats of bringing about "the decline and fall of America."

The U.S. president has major achievements to tout, including curbs on prescription drug pricing, ramped-up microchip manufacturing and record investments in infrastructure.

Democrats have struggled to turn these legislative victories into enthusiasm in the U.S. heartland.

But Amy Klobuchar, a 2016 presidential hopeful, pushed back Sunday on the suggestion Democrats had lost the messaging war, projecting a good night for her party.

Around 48% of likely voters said they prefer a Democratic-controlled Congress in the final national NBC News poll of the campaign, while 47% want Republicans in charge.

But 80% of Republican-leaning voters say they are certain to turn out or have already done so, according to a new Washington Post-ABC News poll, six points above the Democratic figure.

Turnout forecasters always keep a keen eye on Election Day weather, which looks to be warmer than average in most of the country.

About 40 million Americans had cast early votes as of Sunday afternoon, according to the United States Elections Project, narrowly surpassing the figure for 2018.