Republicans have suffered a first setback in the battle for Senate control as Democrats picked up a seat in Colorado.

But the Republicans ousted a Democrat in Alabama and well-known Republicans held on in South Carolina and Texas.

Republicans sought to retain their Senate majority against a surge of Democrats challenging US President Donald Trump’s allies across a vast political map. Both parties saw paths to victory, and the outcome might not be known on election night.

In Colorado, Republican senator Cory Gardner was among the most endangered senators as the state shifted leftwards in the Trump era. Democrat John Hickenlooper, a former governor, won the seat.

John Hickenlooper
Democrat John Hickenlooper won in Colorado (Philip B. Poston /Sentinel Colorado via AP)

“It’s time for a different approach,” Mr Hickenlooper said in an live video message posted on Facebook.

White House confidant Lindsey Graham survived the fight of his political life in South Carolina against Democrat Jamie Harrison, whose campaign stunned Washington by drawing more than 100 million US dollars in small-scale donations. In Texas, senator John Cornyn turned back Democrat MJ Hegar, a former Air Force helicopter pilot, in his hardest-fought election in almost two decades.

Republicans also flipped the seat in Alabama that Democrat Doug Jones had won in a special election as former American football coach Tommy Tuberville was elected in the Trump stronghold.

Tommy Tuberville
Republican Tommy Tuberville has won in Alabama (AP/Butch Dill)

From New England to the Deep South and the Midwest to the Mountain West, Republicans are defending seats in states once considered long shots for Democrats. 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.

Mr Trump loomed large over the Senate races as did Democratic presidential nominee Joe Biden. They swooped into key states, including Iowa, Georgia and Michigan, in the final days of the campaigns. Voters ranked the pandemic and the economy as top concerns, according to AP VoteCast, a national survey of the electorate.

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 because the vice president can break a tie.

Polls closed in key states where some of the nation’s most well-known senators were on the ballot. In Kentucky, Senate majority leader Mitch McConnell fended off Democrat Amy McGrath, a former fighter pilot in a costly campaign, but he acknowledged his Republican colleagues face tougher races.

Mitch McConnell
Mitch McConnell has won re-election in Kentucky (AP/Timothy D. Easley)

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.”

In Georgia, two Senate seats were being contested. They could easily be pushed to a January 5 run-off if no candidate reaches the 50% threshold to win.

The Senate will welcome some newcomers as others retire. In Tennessee, Republican Bill Hagerty won the seat held by senator Lamar Alexander, who is retiring. Republican Cynthia Lummis, the former congresswoman from Wyoming, won the Senate seat opened by retiring Republican senator Mike Enzi.

So far, incumbent senators in less competitive races easily won.

Several Democrats were re-elected, including number two leader Dick Durbin of Illinois, Mark Warner in Virginia and Ed Markey, who survived a primary challenge in Massachusetts. Chris Coons kept the Delaware seat once held by Mr Biden, defeating a Republican who previously promoted the baseless QAnon conspiracy theory.

Among Republicans, Tom Cotton in Arkansas, Ben Sasse in Nebraska, Shelley Moore Capito in West Virginia and James Inhofe in Oklahoma won.