WASHINGTON (AP) — Control of the Senate was a razor-close proposition in Tuesday's election as Republicans fought to retain their majority against a surge of Democrats challenging President Donald Trump's allies across a vast political map.

Both parties saw paths to victory, though the outcome might not be known on election night.

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.

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

In South Carolina, where one of the nation's most well-known senators, Republican Lindsey Graham, was in the campaign of his career against Democrat Jamie Harrison, more than 13,000 votes in one county will have to be counted by hand by Friday's deadline to certify returns.

In North Carolina, where GOP Sen. Thom Tillis was struggling to fend of Democrat Cal Cunningham, the state Board of Elections voted Tuesday to keep four polling places open longer — 45 minutes at most — because they opened late, and that was expected to delay statewide reporting of results.

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 Senate Majority Leader Mitch McConnell, making a final campaign swing Monday in Kentucky. His opponent was Democrat Amy McGrath, a former fighter pilot.

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

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

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

Tillis joined Trump’s rally in Fayetteville, North Carolina, on Monday as Tillis struggled against Cunningham despite the married challenger’s sexting scandal with a public relations strategist.

In South Carolina, Graham acknowledging the tight contest after Harrison raised a whopping $100 million by October, an unheard-of sum for the state. The senator, making TV appeals for cash, said he, too, hit the $100 million mark over the weekend.

Democrats have more than one route to secure the three or four seats needed to capture the majority, and GOP 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 GOP Sen. Cory Gardner because it seems he was heading toward 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 GOP Sen. Martha McSally for the seat once held by the late Republican John McCain.

Even the open seat in Kansas, which hasn't elected a Democrat to the Senate since 1932, was being contested.

The biggest risks to Democrats were in Alabama and Michigan.

Republicans were 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 was in an uphill campaign against Republican Tommy Tuberville, a former Auburn football coach.

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

“We think the numbers are moving,” said Senate Leadership Fund president Steven Law.

Still, voter turnout during the COVID-19 crisis remains key, and volatile, as more Americans than ever are voting.

Both Biden and Trump touched down in Georgia, where the state was seeing a boost of new voters. Georgia's two Senate seats were at stake and could very well push to a Jan. 5 runoff if no candidate reached beyond the 50% threshold.

GOP Sen. David Perdue, the former business executive Trump calls his favorite senator, tried to fend off Democrat Jon Ossoff, another candidate who has benefited from the “green wave” of camoaign donations.

Separately, GOP Sen. Kelly Loeffler faced Republican Rep. Doug Collins, as well as Democrat Raphael Warnock, in a special election for the seat she was appointed to fill after the retirement of GOP Sen. Johnny Isakson.

The Maine race between GOP Sen. Susan Collins and Democrat Sara Gideon was another contest that could push past Election Day if no candidate breaks the 50% threshold. Collins has typically rallied support as a centrist with an independent streak, but the tight contest shows the difficulty GOP senators have appealing to Trump’s most ardent backers while also retaining support from more moderate voters.

The political landscape is quickly changing from six years ago, when most of these senators last faced voters. It’s a reminder of how sharply the political climate has shifted in the Trump era.

In Montana, Republican Sen. Steve Daines was trying to hold off Democrat Steve Bullock, the governor, in a state where Trump was popular. Democrats created an opening by working hard to recruit a well-known candidate in Bullock, who also ran in the party's primary for president.

Iowa Sen. Joni Ernst was fighting for a second term against Democrat Theresa Greenfield. Texas Sen. John Cornyn faced an upstart Democrat, MJ Hegar, in the once solidly Republican state.

And in Alaska, newcomer Al Gross, a doctor, broke state fundraising records in part with viral campaign ads as he took on GOP Sen. Dan Sullivan.

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Find AP’s full election coverage at APNews.com/Election2020.