The battle for power in the Senate has tightened as the Democrats picked up a seat in Colorado, but suffered a setback in Alabama.
Meanwhile, the Republicans held their own in high-profile races in South Carolina, Iowa, Texas and Kansas, dramatically narrowing the political map.
Republicans fought to retain their Senate majority by turning back a surge of Democrats challenging allies of US President Donald Trump, and the Democrats’ various paths to seizing control are growing more limited.
With several contests still too early to call, and one Georgia race heading to a January run-off, the final verdict is expected to drag on.
Democrats gained a seat when ex-governor John Hickenlooper ousted Republican senator Cory Gardner in Colorado, a must-win to flip the Senate, but could not hold on in Alabama, where former American football coach Tommy Tuberville beat senator Doug Jones.
At the same time, several battleground states broke for Republicans. South Carolina, where White House ally Lindsey Graham survived the race of his political career against Jamie Harrison; Texas, as senator John Cornyn held off former Air Force helicopter pilot MJ Hegar; Kansas, with Roger Marshall prevailing over state senator Barbara Bollier, a former Republican; and Iowa, where senator Joni Ernst defeated Democrat Theresa Greenfield.
Senate majority leader Mitch McConnell acknowledged the uncertainty ahead even after he secured a seventh term in Kentucky, fending off Democrat Amy McGrath, a former fighter pilot in a costly campaign.
Mr McConnell said: “We don’t know which party will control the Senate. But some things are certain already. We know grave challenges will remain before us, challenges that could not care less about our political polarisation.
“We know our next president will need to unite the country, even as we all continue to bring different ideas and commitments to the table.”
Mr Trump loomed large over the Senate races as did Democratic presidential nominee Joe Biden. The Trump administration’s handling of the Covid-19 crisis, its economic fallout and the nation’s uneasy mood all seemed to be on the ballot.
Securing the Senate majority will be vital for the winner of the presidency. Senators confirm administration nominees, including the cabinet, and can propel or stall the White House agenda. With Republicans now controlling the chamber by 53-47, three or four seats will determine party control, depending on who wins the presidency because the vice president can break a tie.
Democrats reached deep into states once considered off-limits, contesting seats from New England to the Deep South and the Midwest to the Mountain West. They benefited from an unprecedented onslaught of small-dollar donations flowing to Democratic candidates.
But as votes came in Republicans held their own.
In South Carolina, Mr Harrison stunned Washington by drawing more than 100 million US dollars in small-scale donations. But Mr Graham eventually caught up as he led the Senate through confirmation of Mr Trump’s Supreme Court nominee Amy Coney Barrett, sometimes making pleas for cash during appearances on TV.
“We didn’t get the result at the ballot box that we wanted, but we showed courage and determination,” Mr Harrison said on Twitter. “We brought hope back to South Carolina.”