A pair of Senate runoffs in Georgia that will determine which party controls the chamber has reordered the political universe in the days since the election, influencing nearly every decision by members of both parties, including the refusal of most Republicans to recognize President-elect Joseph R. Biden Jr.’s victory.
Senators are lining up day trips to Atlanta to campaign. Party leaders are carefully calibrating their postelection messages to frame the fight on the ground. And Mr. Biden’s transition team is tailoring its plans to two drastically different outcomes, preparing an ambitious agenda in case Democrats are able to win both races and take control of the Senate, and a more pared-back one in case they fall short.
The most jarring consequence has been Republicans’ refusal to challenge Mr. Trump’s false claims that he won the election. Though most leading Republicans have not repeated his claims, they have also declined to acknowledge Mr. Biden’s clear victory, fearing that doing so would enrage the president and his loyal base of supporters ahead of January.
“We need his voters,” Senator John Thune of South Dakota, the second-ranking Republican, told reporters on Wednesday. “Right now, he’s trying to get through the final stages of his election and determine the outcome there. But when that’s all said and done, however it comes out, we want him helping in Georgia.”
The stakes are staggeringly high for both sides. With Republicans on track to control 50 Senate seats and Democrats 48, the twin runoffs on Jan. 5 will determine how much power Mr. Biden can wield in a post-Trump Washington.
The contests pit two Republican senators, David Perdue and Kelly Loeffler, against the Democratic challengers Jon Ossoff and the Rev. Raphael Warnock. But they are quickly becoming nationalized into referendums on Mr. Biden’s victory and the direction of the country.
The impact was apparent throughout the Capitol. Speaker Nancy Pelosi, Democrat of California, privately warned House members to watch their words in the coming weeks to deny Republicans any new ammunition to caricature the Democrats as extremists.
Senator Mitch McConnell of Kentucky, the majority leader, bucked up Mr. Trump and expressed outrage over Democrats insisting the president concede.
Senator Marco Rubio of Florida was on the ground Wednesday in the Atlanta suburbs rallying with Ms. Loeffler at a Save Our Majority Rally. Senator Rick Scott, the newly elected chairman of Senate Republicans’ campaign arm, is scheduled to hold a fund-raiser in the state on Thursday.
And Vice President Mike Pence told senators privately on Tuesday that he planned to jet down to Georgia next week to campaign.
Democrats were seeking to rally their own voters and donors in a long-shot bid to win both races in a historically conservative state where their party has often fallen short in runoff contests.
“When it comes to the Senate, it’s not over, at all,” Senator Chuck Schumer of New York, the minority leader, said on Wednesday. “Georgia is close.”