Candidates and big-name backers made final appeals to voters in the last hours of a fraught midterm election season.
Republicans were excited about the prospect of winning back Congress while President Joe Biden insisted his party would “surprise the living devil out of a lot of people”.
Democrats contend Republican victories could profoundly and adversely reshape the country, eliminating abortion rights nationwide and unleashing broad threats to the very future of American democracy.
Republicans say the public is tired of Mr Biden’s policies amid high inflation and concerns about crime.
“We know in our bones that our democracy is at risk,” Mr Biden said during an evening rally in Maryland, where Democrats have one of their best opportunities to reclaim a Republican-held governor’s seat.
“I want you to know, we’ll meet this moment.”
Arriving back at the White House a short time later, Mr Biden was franker, saying: “I think we’ll win the Senate. I think the House is tougher.”
Asked what the reality of governing will be like, he responded: ”More difficult.”
The Maryland event followed Mr Biden’s late-campaign strategy of sticking largely to his party’s strongholds rather than stumping in more competitive territory, where control of Congress may ultimately be decided.
Mr Biden won Maryland with more than 65% of the vote in 2020 and appeared with Wes Moore, the 44-year-old Rhodes Scholar who could become the state’s first black governor.
The president said at an earlier virtual event: “Imagine what we can do in a second term if we maintain control.”
Most political prognosticators do not think the Democrats will — and predict that Tuesday’s results will have a major impact on the next two years of Mr Biden’s presidency, shaping policy on everything from government spending to military support for Ukraine.
In the first national election since the January 6, 2021 Capitol riot, the Democrats have tried to focus key races on fundamental questions about the nation’s political values.
The man at the centre of most January 6 debate, former president Donald Trump, was in Ohio for his final rally of the 2022 campaign — and already thinking about his own future in 2024.
He had teased that he might formally launch a third presidential run at Monday night’s rally with Senate candidate JD Vance — which Mr Trump concluded by promising a “big announcement” next week at his Mar-a-Lago estate in Florida.
“If you want to stop the destruction of our country and save the American dream, then tomorrow you must vote Republican in a giant red wave that we’ve all been hearing about,” Mr Trump said at Monday night’s rally.
He also went after House Speaker Nancy Pelosi, saying “I think she’s an animal” mere days after her husband, Paul, was severely beaten by an attacker at the couple’s San Francisco home.
First lady Jill Biden appeared with her husband in Maryland but also campaigned earlier on Monday for Democratic Representative Jennifer Wexton in northern Virginia.
The first lady told about 100 people outside a home in Ashburn, about 30 miles from Washington, that the race could come down to a tiny margin of votes. And she warned that, in Congress, a “Republican majority will attack women’s rights and health care”.
Mr Trump’s backing of Mr Vance in Ohio this year was crucial in helping the author and venture capitalist — and onetime Trump critic — secure the GOP’s nomination for a Senate seat. He is now facing Democrat Tim Ryan.
“When I think about tomorrow, it is to ensure the American dream survives into the next generation,” Mr Vance declared to thousands of cheering supporters, some sporting Trump 2024 hats and T-shirts, at Dayton International Airport.
While the GOP likes its chances of flipping the House, control of the Senate could come down to a handful of crucial races. Those include Georgia, Arizona and Pennsylvania, where Democratic Governor John Fetterman was locked in a close race against Republican celebrity surgeon Mehmet Oz.
In Georgia, Democratic senator Raphael Warnock, who was in a nail-biter with Republican Herschel Walker, tried to cast himself as pragmatic — capable of succeeding in Washington even if the GOP has more power.
Arizona Democratic senator Mark Kelly also tried to strike a moderate tone. He praised the state’s late Republican senator, John McCain, while noting that he did not ask Mr Biden to campaign with him but would “welcome the president to come here at any point”.
Mr Kelly’s Republican rival, Blake Masters, called the senator “just a rubber stamp vote for Joe Biden’s failed agenda”.
Elon Musk, whose purchase of Twitter has roiled the social media world, used that platform on Monday to endorse the GOP, writing: “I recommend voting for a Republican Congress, given that the Presidency is Democratic.”
That came too late for more than 41 million Americans who had already cast early ballots.