Democrats took control of the Senate on Wednesday with a pair of historic victories in Georgia’s runoff elections, assuring slim majorities in both chambers of Congress for President-elect Joseph R. Biden Jr. and delivering an emphatic, final rebuke to President Trump in his last days in office.
The Rev. Raphael Warnock defeated Senator Kelly Loeffler, becoming the first Black Democrat elected to the Senate from the South. And Jon Ossoff, the 33-year-old head of a video production company who has never held public office, defeated David Perdue, who recently completed his first full term as senator.
Both Democrats now lead their defeated Republican opponents by margins that are larger than the threshold required to trigger a recount under Georgia law.
The Democrats’ twin victories will reshape the balance of power in Washington. Though they will have the thinnest of advantages in the House and Senate, where Vice President-elect Kamala Harris will break 50-50 ties, Democrats will control the committees and the legislation and nominations brought to the floor. That advantage will pave the way for at least some elements of Mr. Biden’s agenda.
Mr. Ossoff’s victory comes at a moment when the nation’s political leadership has been paralyzed by a pro-Trump mob that stormed the Capitol and halted the formal accepting of the Electoral College results by Congress. The day’s extraordinary proceedings — rioting interrupting the peaceful transition of political power — crystallized the campaign the Georgia Democrats ran against their Republican opponents, both of whom pledged to seek to overturn the results of the presidential election to keep Mr. Trump in office.
The Republicans’ losses in a state that Mr. Biden narrowly carried in November, but that still leans right politically, also amounted to a vivid illustration of the perils of embracing Mr. Trump. He put his diminished political capital on the line with an election eve appearance in Northwest Georgia. And Mr. Perdue and Ms. Loeffler unwaveringly embraced the president throughout the runoff races even as he refused to acknowledge Mr. Biden’s victory and brazenly demanded that Georgia state officials overturn his loss in the state.
The political fallout of Mr. Trump’s tenure is now clear: His single term in the White House will conclude with Republicans having lost the presidency, the House and the Senate on his watch.
Mr. Ossoff and Mr. Warnock won thanks to a frenetic get-out-the-vote push that began immediately after the November election, when no candidate in either race claimed the majority needed to avoid a runoff. Driving turnout among liberals and Black voters in the early-voting period, Democrats built an insurmountable advantage going into election day.
They won thanks to overwhelming margins in Georgia’s cities, decisive victories in Georgia’s once-Republican suburbs and because of lackluster turnout on Tuesday in the rural counties that now make up the G.O.P. base.
The Rev. Raphael Warnock, a Democrat and the pastor at the historic Ebenezer Baptist Church in Atlanta, defeated Senator Kelly Loeffler, a Republican, to become the first Black senator in Georgia history and the first Black Democrat to be elected to the Senate in the South.
In the other contest, Jon Ossoff, the Democratic challenger, defeated David Perdue, the Republican whose Senate term ended on Sunday.
Earlier on Wednesday morning, before major networks called the race, Mr. Ossoff declared victory and Senator Chuck Schumer of New York, the Democratic leader, proclaimed that his party would win the majority.
“It feels like a brand new day,” Mr. Schumer said in a statement. “For the first time in six years, Democrats will operate a majority in the United States Senate — and that will be very good for the American people.”
Mr. Perdue has not yet conceded the race but Gabriel Sterling, a top Georgia elections official, said late Wednesday morning that Mr. Ossoff would most likely win by a margin large enough to avoid a recount, which is 0.5 percent in Georgia.
With Democrats winning both races, Mr. Biden now has a stronger ability to enact his agenda. Democrats will hold 50 seats in the Senate and de facto control of the chamber, with Vice President-elect Kamala Harris serving as the tiebreaking vote and Senator Mitch McConnell relegated to minority leader.
Mr. McConnell, the Republican leader who had indulged Mr. Trump’s baseless accusations of election fraud for the last two months, publicly broke with him after losing one race in Georgia and falling behind in the other. In a floor speech before pro-Trump protesters breached the building, he called rejecting efforts to overturn the election the most important vote of his decades-long career and warned of sending democracy into a “death spiral.”
“We simply cannot declare ourselves a national board of elections on steroids,” Mr. McConnell said. “The voters, the courts, and the states have all spoken. They’ve all spoken. If we overrule them, it would damage our Republic forever.”
Not long after, Trump supporters put the proceedings on hold by making their way inside the building by force — including the Senate chamber itself. Inside the House chamber, photos showed law enforcement officers with guns drawn.
Even before the Georgia results were official, the Republican recrimination began about how the party not only lost the White House but was also at risk of ceding control of the Senate.
“It turns out telling voters the election is rigged is not a good way to turn out your voters,” said Senator Mitt Romney, Republican of Utah and an outspoken critic of Mr. Trump.
Later, Mr. Romney, from a secure location after the breach, declared of the chaos at the Capitol, “This is what the president has caused today, this insurrection.”
The twin Georgia races drew record levels of campaign spending — roughly half a billion dollars in two months — and national attention, with Mr. Trump and Mr. Biden both campaigning in the state on Monday.
The results showed both Mr. Warnock and Mr. Ossoff carrying a larger share of the vote in county after county — particularly in majority-Black areas — than Mr. Biden did in November, when he became the first Democratic presidential candidate to win Georgia since 1992.