Republicans are fighting to keep control of the Senate in a close contest against a surge of Democrats challenging US President Donald Trump’s allies.

Polls started closing in key states where some of the nation’s most well-known senators and hotly contested races 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.

It could be a long wait, as both parties saw paths to victory, and the outcome might not be known on election night.

Republican senator Lindsey Graham is in 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. More than 13,000 votes in one county will be delayed and have to be counted by hand by Friday’s deadline to certify returns.

Polls also closed in Georgia, where two Senate seats were being contested. They could easily push to a January 5 run-off if no candidate reaches the 50% threshold to win.

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. Their campaigns 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.

“Let’s run through the tape,” said Mr McConnell, making a final campaign visit on Monday in Kentucky.

Mr McConnell said he hoped to remain majority leader alongside Mr Trump, but acknowledged the tough Senate races could flip control to the Democrats.

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 senator Mike Enzi.

The campaigns were competing across an expansive map as Democrats put Republicans on defence deep into Trump country.

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.

Stuck in Washington to confirm Mr Trump’s Supreme Court nominee Amy Coney Barrett a week before polling day, senators quickly fanned out – some alongside the president – for last-ditch tours, often socially distanced in the pandemic, to shore up votes.

Republican Thom Tillis joined Mr Trump’s rally in Fayetteville, North Carolina, on Monday as Mr Tillis struggled against Democrat Cal Cunningham despite the married challenger’s sexting scandal with a public relations strategist. Mr Cunningham travelled around the state on Tuesday, talking to voters in Efland, near Durham.

The state Board of Elections voted on Tuesday to keep four North Carolina polling places open longer – 45 minutes at most – because they opened late, and that was expected to delay state-wide reporting of results.

Democrats have more than one route to secure the three or four seats needed to capture the majority, and Republican strategists privately acknowledged that the incumbents will almost certainly suffer defeats in some races.

Younger voters and more minorities are pushing some states toward Democrats. In Colorado, the parties have essentially stopped spending money for or against Republican senator Cory Gardner because it seems he was heading towards defeat against Democrat John Hickenlooper, a former governor.

Arizona could see two Democratic senators for the first time since last century if former astronaut Mark Kelly maintains his advantage over Republican senator Martha McSally for the seat once held by the late Republican John McCain.

Even the open seat in Kansas, which has not elected a Democrat to the Senate since 1932, is being contested.

The biggest risks to Democrats are in Alabama and Michigan.

Republicans are expecting to reclaim the seat in Alabama, where Democrat Doug Jones pulled off a rare 2017 special election win in the Trump stronghold. Now, however, he is in an uphill campaign against Republican Tommy Tuberville, a former American football coach.

In the presidential battleground of Michigan, Republicans have made an aggressive push for John James, a black Republican businessman, against Democratic senator Gary Peters.

“It’s my honour to stand before you today, Michigan,” Mr James said at a final rally late on Monday on stage with the president. “It’s time for a change.”

As voters were still at the polls, Mr Peters stayed on message on Tuesday tweeting, “Today, health care is on the ballot.”