Democrats’ hopes are fading for control of the US Senate as Republicans repelled an onslaught of challengers and fought to retain their fragile majority.

Several races remain undecided heading into Wednesday morning, with at least one heading to a run-off vote in January.

This was a disappointing outcome for Democrats who had devised an expanded political map, eager to provide a backstop against US president Donald Trump and his party’s grip on the Senate.

The races attracted an unprecedented outpouring of small-dollar donations from Americans apparently voting with their wallets to propel long-shot campaigns.

The voters’ choices will likely force a rethinking of Democratic Party strategy, messaging and approach.

While Democrats picked up must-win seats in Colorado and Arizona, they suffered a setback in Alabama, and Republicans held their own in one race after another – in South Carolina, Iowa, Texas, Kansas and Montana, dramatically limiting the places where Democrats hoped to make inroads.

“You wasted a lot of money,” said White House ally Lindsey Graham in Columbia, South Carolina, after defeating Jamie Harrison, despite the Democrat’s stunning 100 million dollar (£77 million) haul for his campaign.

“This is the worst return on investment in the history of American politics.”

Voters had ranked the coronavirus pandemic and the economy as top concerns, according to AP VoteCast, a national survey of the electorate.

“It’s time for a different approach,” said Democrat John Hickenlooper, a former governor who unseated the Republican incumbent in Colorado, during a live video message posted on Facebook.

Yet voters, for the most part, stuck with the status quo.

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, 53-47, three or four seats will determine party control, depending on who wins the presidency, as the vice president can break a tie in the Senate.

Democrats contested seats from New England to the Deep South and the Midwest to the Mountain West, reaching deep into Republican strongholds. But by early on Wednesday, the tally was not too different from before US election day.

The Democrats’ gains were in Colorado and Arizona, where former astronaut Mark Kelly beat Republican incumbent Martha McSally. But they could not hold on in Alabama, where former college football coach Tommy Tuberville defeated Doug Jones.

Several battlegrounds broke for Republicans. In South Carolina, Mr Graham survived the race of his political career against Mr Harrison. In Texas, John Cornyn turned back former Air Force helicopter pilot MJ Hegar; in Iowa, Joni Ernst defeated Democrat Theresa Greenfield; in Montana, Steve Daines routed Steve Bullock; and in Kansas, Roger Marshall prevailed over Barbara Bollier, a former Republican who energised Democrats in a state that has not elected a Democrat to the Senate since 1932.

The final breakdown awaits the outcome of races in Alaska, Maine, Michigan and North Carolina.

Senate majority leader Mitch McConnell acknowledged the uncertainty still ahead even after he secured a seventh term in Kentucky, fending off Democrat Amy McGrath, a former fighter pilot in a costly campaign.

“We don’t know which party will control the Senate,” Mr McConnell said from Louisville.

“But some things are certain already. We know grave challenges will remain before us, challenges that could not care less about our political polarization. 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.”

In Georgia, two seats were being contested and at least one is headed to a run-off after no candidate reached the 50% threshold to win.

Republican senator Kelly Loeffler will face Democrat Raphael Warnock in the January 5 run-off special election for the seat Ms Loeffler was tapped to fill for the retired Johnny Isakson.