WASHINGTON (AP) — Control of the Senate won’t be decided until January runoffs in Georgia after neither party Wednesday locked down a majority, launching a mammoth battle to shape President-elect Joe Biden's agenda and determine the balance of power in Washington.

The deadlock became official after Republicans held the Senate seat in Alaska. There, incumbent GOP Sen. Dan Sullivan defeated Al Gross, an independent running as a Democrat, after an onslaught of mail-in ballots delayed counting until Tuesday, a week after Election Day. A short while later, North Carolina GOP Sen. Thom Tillis defeated Democratic challenger Cal Cunningham.

Democrats are now denied an immediate path to the majority, but Republicans are also short. Instead, the sprint to the Jan. 5 runoffs for two seats in Georgia will determine whether the Senate becomes a Republican-held check on Biden's agenda or a Democratic partnership with the new White House.

“We’ve got to go win Georgia,” said Sen. Rick Scott, R-Fla., the incoming chairman of the National Republican Senatorial Committee, after he was elected by his colleagues Tuesday.

Democrats are amassing an army of volunteers, fueled by a fresh onslaught of donations from Americans nationwide eager to ensure the Senate, like the Democratic-majority House, has Biden's back.

The “entire Democratic infrastructure” is going to focus on winning Georgia, said one aide, granted anonymity to discuss the situation.

With Biden’s victory, Republicans would need 51 seats for majority control, since the vice president of the party in the White House — soon to be Kamala Harris — becomes a tie-breaker in the Senate.

As the tally now stands, Republicans will have a 50-48 hold on the Senate heading into the new Congress.

The Senate can make or break the White House's agenda. With a Democratic Senate, Biden would have allies to easily confirm his nominees, including for cabinet positions, and shape passage of legislation. If Republicans keep control, Senate Majority Mitch McConnell can block Biden's agenda.

It's unclear how heavily President Donald Trump will fight to keep the Senate in Republican hands after his own election defeat.

As Trump refuses to accept the outcome of the presidential election, waging a legal battle with unsubstantiated claims of voter fraud, his GOP allies in the Senate are following his lead. Senate Republicans need Trump — and his voters — on board for the Georgia race. Privately, though, many Republicans doubt Trump has a path to overturning Biden's victory.

The political world swiftly focused on Georgia, where the campaigns have already begun.

GOP Sen. Kelly Loeffler will face Rafael Warnock, a Black pastor from the church where Rev. Martin Luther King, Jr. preached. And Republican Sen. David Perdue, a top Trump ally, will face Democratic challenger Jon Ossoff.

Warnock, in an interview with MSNBC, brushed back early attack ads and vowed to go to Washington “to stand up for ordinary people.”

Strategists estimate an eye-popping $500 million could pour into the state in coming weeks. Georgia is a politically divided state, with Democrats making gains on Republicans, fueled by a surge of new voters. But no Democrat has been elected senator in some 20 years.

On Wednesday, GOP Sen. Marco Rubio, R-Fla., joined Loeffler and Perdue’s wife, Bonnie, for crowded, indoor rally, despite the surging COVID-19 crisis, as Republican senators join the push to salvage their majority.

Democrats immediately blasted Perdue for skipping the event.

Loeffler and Perdue stunned many this week when they jointly called their own state election system an “embarrassment" as Biden was leading over Trump. Georgia's Secretary of State on Wednesday announced an audit of presidential election results that he said would be done with a full hand tally of ballots because the margin is so tight.

Both parties suffered defeats in the election for Senate that defied expectations. Their majority shrunk, Republicans were defeated in Arizona and Colorado as Democrats added to their ranks. But Democrats lost one of their own in Alabama and failed to win as they took on other Republicans they targeted across a wide map.