The battle for control of the Senate headed into a two-month overtime as Georgia prepares to host two runoff elections Jan. 5 that are likely to determine which party controls the chamber, with the potential to dramatically alter the arc of a new Democratic administration if Joe Biden wins the presidency.

As the dust settled on almost every race, Republicans have secured 48 seats in next year’s Senate and hold steady leads in two other contests but need to win at least one of the two races in Georgia to land a clear majority of 51 seats.

That leaves Democrats, with a caucus of 48 senators so far, one last chance to reclaim the majority by trying to secure a double victory in what used to be a conservative Republican stronghold. If successful, and if Biden secures the White House, the 50-50 Senate would tilt to the Democrats once Sen. Kamala D. Harris (Calif.) is sworn in as vice president.

The stakes set the stage for an unfathomable amount of money pouring into a state that has already witnessed an estimated $150 million in advertising in the initial Senate campaigns, now serving as the only two races left and poised to determine the majority.

All four campaigns, and their various outside supporters, expect to try to nationalize the race and focus their messaging on the impact that victories could have for each side, with Democrats trying to achieve a historically high Black turnout normally associated with a presidential race.

Raphael Warnock, seeking to become the first elected Black Democratic senator from the Deep South, faces Sen. Kelly Loeffler (R) in a race that was long expected to go to a runoff because it is a special election to fill the remainder of the term won four years ago by Johnny Isakson (R). Loeffler was appointed to the seat after Isakson resigned for health reasons at the end of last year.

But by Thursday afternoon the state got the double-bonus when Sen. David Perdue (R), running for a second term, fell below the 50 percent threshold that is required by Georgia law to win a Senate race. If his share of the vote remains below that marker, Perdue would face Democrat Jon Ossoff, a documentary filmmaker running his first statewide race.

Ossoff’s campaign expressed confidence even as Perdue was likely to finish this round about two percentage points ahead.

“We are confident that Jon Ossoff’s historic performance in Georgia has forced Senator David Perdue to continue defending his indefensible record,” Ellen Foster, campaign manager for Ossoff, said in a statement.

Republicans are ready to paint the races as securing a GOP majority to shore up the conservative bulwark against liberal Democrats, particularly if there is an incoming Biden administration.

They said that the best chance Ossoff and Warnock had came on this first ballot when turnout was so high for the presidential race, calling Ossoff a “perennial loser” who already lost a special election for a House seat in 2017. Republicans expect turnout for an oddly timed, post-Christmas election to be a more conservative collection that makes their path clearer to securing the Senate majority.

“If Ossoff couldn’t do it now, he’s not going to be able to do it in two months when even more is at stake,” said Jesse Hunt, spokesman for the National Republican Senatorial Committee, as both parties prepared for the second runoff.

These Georgia races give Democrats the chance to overcome the disappointment of Tuesday’s losses in a handful of races they considered tossups that appeared to leave them still in the minority for a fourth successive election.

Moreover, these two Senate races will serve as a rescue effort to the ambition of a new Biden administration, should he win the White House. If Senate Majority Leader Mitch McConnell (R-Ky.) remains in power, every Cabinet choice of the new administration would face deep scrutiny and any deemed too liberal would probably not be confirmed.

A GOP-controlled Senate will continue to ignore the liberal legislation moved out of Speaker Nancy Pelosi’s House.

And, should a vacancy arise, Biden’s Supreme Court choices would need bipartisan blessing to win confirmation.

Traditionally, Republicans hold an advantage when Georgia’s statewide races go to the runoff phase, including winning two statewide offices in such races two years ago. The last Senate race sent to a runoff, in 2008, ended with then-Sen. Saxby Chambliss (R) winning by almost 15 percentage points after a general-election race in which the incumbent finished with a slender margin of three percentage points.

But Democrats argue that the state has been transformed dramatically in the past five years, first because the metro Atlanta region has been an economic draw for diverse newcomers. And, after Democrat Stacey Abrams’s narrow loss in the 2018 gubernatorial election, she formed Fair Fight, a group designed to increase Democratic voter registration particularly among Black residents.

Democrats estimate that there are 800,000 new voters on the state’s rolls in the last two years.

And Biden’s performance only buoyed hopes of breaking through with down-ballot victories as well. After giving an initial large lead for President Trump, the state began counting mail votes that tilted heavily toward the former vice president and, as of Thursday evening, narrowing Trump’s lead to less than 10,000 votes, almost a statistical tie.

All four candidates must now reboot their campaigns for the new race, without any letup after a grueling stretch through most of the year. Local election officials can begin sending out mail ballots Nov. 18, and early in-person voting begins Dec. 14.

Warnock, the top minister at Ebenezer Baptist Church, where the Rev. Martin Luther King Jr. once preached, recognized that he has had the easiest run of the candidates so far.

“Get ready, Georgia, the negative ads are coming. Kelly Loeffler doesn’t want to talk about why she’s for getting rid of health care in the middle of a pandemic. So she’s going to try to scare you with lies about me,” Warnock said Thursday morning in a video he released, to preempt the likely barrage of negative ads heading toward him.

Because of the unique nature of his special election, Warnock has not faced any real attack ads yet, as all outside GOP groups focused all their fire on Ossoff in the unsuccessful effort to boost Perdue above 50 percent on the initial vote.

Also, Loeffler first had to fend off a challenge on her right flank from Rep. Douglas A. Collins. Those two had a bitter fight to become the top Republican vote-getter to advance to the runoff, as Warnock faced only nominal opposition from a few underfunded Democrats.

Loeffler, a first-time politician from the moderate GOP haven of Atlanta’s wealthy Buckhead neighborhood, moved sharply to her right to prove conservative bona fides up against Collins, one of the president’s top defenders in the House’s 2019 impeachment proceedings.

Asked during a debate if she disagreed with Trump on a single issue, Loeffler had a blunt response: “No.”

Some Republicans now privately worry that Loeffler has left open the middle ground of independents and moderate Republicans who have grown disenchanted with Trump’s grievance-focused presidency.

On Thursday, Republicans made clear that they will try to tie both Warnock and Ossoff to liberal activists who pushed the “defund the police” movement in the wake of several police brutality allegations over the summer.

“Warnock has openly talked about punitively cutting police budgets. Ossoff said funding for police departments ‘should be on the line,’ ” Matthew Whitlock, a senior NRSC adviser, tweeted.