Within minutes of Joe Biden becoming president-elect, top Democrats and Republicans raced to the front lines of 2020’s last battlefront: a pair of January Senate runoffs in Georgia where the country’s racial, economic and cultural crosscurrents could help determine whether Democrats complete their takeover of Washington.

Republicans looking to turn the page on President Trump’s defeat shifted their attention to the runoffs over the weekend, framing them as a last line of defense against a left-wing agenda. Democrats, seeking to capitalize on their momentum and celebratory mood, promoted the races as the best way to advance Biden’s policies.

That makes the Jan. 5 runoffs an unusual finale to a tempestuous campaign rocked by a deadly pandemic, a national reckoning on race and an economic free fall. The races will unfold in a rapidly diversifying state that has become a national bellwether, one whose votes split nearly evenly between Biden and Trump.

The Post’s Amy Gardner explains how a once reliably red Georgia underwent a political transformation this election year due to the work of Stacey Abrams. (The Washington Post)

“I don’t think it’s possible to overstate how big these races are,” said Nse Ufot, chief executive of the New Georgia Project, which focuses on mobilizing young voters of color.

After netting one Senate seat last week, Democrats need to flip two more to get to 50, which would give them effective control of the chamber because Kamala D. Harris could cast tie-breaking votes as vice president.

One of the races in Georgia pits Sen. David Perdue (R), a close Trump ally who drew accusations of racism when he mispronounced Harris’s name recently, against Democrat Jon Ossoff, a 33-year-old investigative journalist and former House candidate.

The other contest is a special election that features Sen. Kelly Loeffler (R), who was appointed by Gov. Brian Kemp (R) to fill a vacant seat, facing a challenge from pastor Raphael Warnock, who would be the first elected Black Democratic senator from the Deep South. Runoffs are triggered in Georgia when no candidate receives more than 50 percent of the vote.

Behind the scenes, both sides are already plotting strategy. Advisers to Biden and Senate Minority Leader Charles E. Schumer (D-N.Y.) have been in touch about the campaign, according to two people with knowledge of the situation, who spoke on the condition of anonymity to describe private interactions.

But questions remain about how engaged Biden and his campaign operation will be in the races, which will overlap with Biden’s transition to the White House. Diving deep into a polarizing showdown could complicate his bid to bring the country together after a divisive election.

What’s more, a strategic defeat two weeks before his inauguration could weaken Biden politically. Some close allies suggested Biden and his team should move cautiously when it comes to, as one of them put it, “chasing the white whales.”

But other supporters insisted the Georgia races must be a top priority for Biden’s operation, even if he is not the public face of the campaign. From Cabinet appointments to Supreme Court nominations to legislation to international treaties, the difference between a Republican and Democratic Senate could determine the fate of his agenda.

Sen. Christopher A. Coons (D-Del.), a close Biden friend, said he has already received texts from a family member in Georgia expecting legions of Democratic foot soldiers for the runoffs. “They’ve got the guest room ready,” Coons said. “I think you’ll see a lot of engagement.”

A Biden aide said recently that it was too soon to discuss his team’s specific plans but that there was no reason not to engage aggressively in Georgia. The aide spoke on the condition of anonymity because the individual was not authorized to speak publicly.

The races are expected to draw record sums of cash, a flood of TV commercials and large armies of surrogates, as well as get-out-the-vote operations fine-tuned to deal with the dual challenge of campaigning during the holiday season and a coronavirus outbreak. Democrat Stacey Abrams, who in 2018 came close to winning the Georgia governor’s mansion, is encouraging voters to request absentee ballots, and Democrats probably will lean heavily on the network and strategy she developed in her gubernatorial race.

“This is going to be the determining factor of [whether] we have access to health care and access to justice in the United States — those are two issues that will make certain that people turn out,” Abrams said Sunday on CNN’s “State of the Union.” “We know this is going to be a hard fight. It’s going to be a competitive fight.”

Each side is also seeking to apply the lessons learned during a presidential election in which Biden and Trump fought to a near-draw in Georgia. Biden led Trump by two-tenths of a percentage point with most ballots tallied Sunday. “With a margin that small, there will be a recount,” Georgia Secretary of State Brad Raffensperger, a Republican, said last week.

“The people of Georgia will speak again on January 5, 2021,” Warnock tweeted Sunday.

Few states embody the forces charging the country’s political and social debates the way Georgia does. It was in Georgia where Ahmaud Arbery, an unarmed Black jogger, was fatally shot by a White former police officer, fueling months of nationwide street protests.

Georgia was also the state where the first open supporter of the baseless QAnon conspiracy theory, Marjorie Taylor Greene, won a seat in Congress. Greene has made racist statements at a time when many African Americans feel the GOP has become more hospitable to white supremacists.

In general, the state’s political geography is a microcosm of the nation’s political landscape. Trump was propelled in Georgia, as he was across the country, by strong support from White and rural voters, while Biden performed strongly among suburban voters and with African Americans and city dwellers.

If Biden wins Georgia, he will be the first Democratic presidential hopeful since Bill Clinton in 1992 to carry the state. He competed aggressively in Georgia, traveling there in the final stretch and dispatching former president Barack Obama to help.

Democrats’ disappointing showing last week in other Senate contests has narrowed the party’s path to reclaiming the chamber, meaning that Sen. Mitch McConnell (R-Ky.), who is loathed by many Democrats, could stay on as majority leader. McConnell’s allies responded Saturday to Biden’s victory by using it as a jumping-off point for the Georgia runoffs.

“A Democratic majority in the Senate would cinch the radical agenda of the left. They must be stopped on January 5th in Georgia,” tweeted Sen. John Cornyn (R-Tex.), who just won reelection. Sen. Tom Cotton (R-Ark.), a potential 2024 presidential hopeful, tweeted that the Republican candidates in Georgia “need all hands on deck.”

Schumer, whose hopes of becoming majority leader have faded because of Democratic losses in states such as Maine, also turned toward Georgia on Saturday.

“Joe Biden will win Georgia and so many other states because his agenda brings America together and helps working families,” he said in a statement, adding, “The best way to ensure that positive agenda can be carried out and deliver help to working families in Georgia and across the country is to elect Raphael Warnock and Jon Ossoff to the Senate.”

Ossoff’s campaign is seeking to capitalize on the momentum that Democrats have built in recent months, which helped generate high enthusiasm in the suburbs of Atlanta. His campaign officials are also confident about raising large sums of money.

But record-shattering fundraising was not enough to put other Democratic Senate candidates over the top on Nov. 3, and Republicans in Georgia say they will have adequate resources to compete effectively. They sought to attract new money on Saturday, seizing on a clip of Schumer saying, “Now we take Georgia, then we change America.”

“Not on our watch,” Perdue tweeted in response. “Donate now to help me and @KLoeffler defeat Chuck Schumer. We win these two races, we save the senate. We save the Senate, we save the country. This is what is at stake.”

Still, Republicans are searching for the right blueprint for the state. Seth Weathers, a Republican consultant in Georgia who oversaw the state for Trump’s campaign in 2016, said the party needs to “figure out the suburbs” if it wants to keep the state red. “It was pretty brutal,” Weathers said of suburban voters’ defections from Trump.

At the same time, Weathers noted that Trump’s style and message solidified support in rural parts of the state. “It makes it quite the conundrum,” he said.

It’s not yet clear whether Trump himself plans to engage in the Georgia runoffs and whether his base will turn out for an election when he is not on the ballot. But much of Georgia remains highly conservative, and Trump’s absence might make it easier for Perdue and Loeffler to gain ground in the suburbs.

Ossoff will begin a statewide tour Tuesday, the day the Supreme Court hears arguments against the Affordable Care Act. That is a calculated choice, since the campaign plans to highlight Republican struggles to manage health care and the coronavirus, a message that was a staple of Biden’s campaign.

Ossoff launched his first ad of the runoff on Saturday, a spot that talked about recovering from the pandemic and echoed Biden’s unity themes. Warnock has also released an ad that humorously warns voters about the expected negative attacks against him.