Democrats Raphael Warnock and Jon Ossoff defeated Georgia’s two sitting Republican senators on Tuesday, handing Democrats complete control of Congress.
Their wins in the run-off elections also gave president-elect Joe Biden more leeway in confirming his Cabinet nominees and pursuing his legislative agenda.
Their victories also ensure that vice president-elect Kamala Harris will assume an unusually active role on Capitol Hill.
It will fall to Ms Harris to cast any tie-breaking votes in the Senate, which will be split between 50 Democrats and 50 Republicans.
This will be the fourth time in the US history that a vice president has presided over an evenly split Senate.
The Senate was last split 50-50 between the parties in 2001, with former vice president Al Gore ensuring Democratic control for less than a month before Republicans took control when former vice president Dick Cheney assumed office.
Because of Georgia’s election laws, the two Republican incumbents – Kelly Loeffler and David Perdue – had to face off against Mr Warnock and Mr Ossoff as none of the candidates won more than 50 per cent of the vote in the November elections.
While campaigning for Mr Warnock and Mr Ossoff on Monday, Mr Biden vowed that $2,000 stimulus cheques “will go out the door” if they won their races and secured Democratic control of Congress.
While President Donald Trump had also endorsed directly paying Americans the same amount, the economic stimulus package he signed last month contained only $600 cheques because legislators in his party were unwilling to provide the $2,000.
Ms Loeffler and Mr Perdue, who tied themselves closely to Mr Trump, came out in favour of $2,000 stimulus cheques in the final days of the race.
Mr Biden and the Democrats also condemned Mr Trump over a released recording of a call on Saturday in which he pressured Georgia Secretary of State Brad Raffensperger to "find" exactly enough votes to undo his November loss in the state.
Ms Loeffler and Mr Perdue joined Mr Trump’s earlier attacks on Mr Raffensperger, echoing the president’s unsubstantiated claims of voter fraud.
Mr Trump made the same allegations about Tuesday’s run-off election on Twitter several hours before the Democratic victories.
Fair Fight Action, a non-profit organisation founded by politician Stacey Abrams to advocate changes in voter registration, is largely credited with the unusually high level of Democratic turnout and involvement in a special election.