Georgia voters are set to decide the balance of power in the US Congress in a pair of high-stakes Senate runoff elections that will help determine Joe Biden’s capacity to enact what may be the most progressive governing agenda in generations.

Republicans are unified against the president-elect’s plans for health care, environmental protection and civil rights, but some fear that outgoing president Donald Trump’s brazen attempts to undermine the integrity of the nation’s voting systems may discourage voters in the state.

At a rally in north-west Georgia on Monday evening, Mr Trump repeatedly claimed the November elections were plagued by fraud that Republican officials, including his former attorney general and Georgia’s elections chief, say did not occur.

The president called Georgia’s Republican secretary of state “crazy” and vowed to help defeat him in two years. At the same time, Mr Trump encouraged his supporters to show up in force for Georgia’s Tuesday contests.

“You’ve got to swarm it tomorrow,” he told thousands of cheering supporters, downplaying the threat of fraud.

Democrats must win both of the state’s Senate elections to gain the Senate majority. The Senate would be equally divided 50-50, with vice president-elect Kamala Harris serving as the tie-breaker for Democrats.

Democrats secured a narrow House of Representatives majority and the White House during November’s general election.

Even a closely divided Democratic Senate is not likely to guarantee Mr Biden everything he wants, given Senate rules that require 60 votes to move most major legislation.

But if Democrats lose even one of Tuesday’s contests, he would have little scope for his most ambitious plans to expand government-backed health care coverage, strengthen the middle class, address racial inequality and combat climate change.

A Republican-controlled Senate also would create a rougher path for Mr Biden’s cabinet picks and judicial nominees.

“Georgia, the whole nation is looking to you. The power is literally in your hands,” Mr Biden said at his own rally in Atlanta earlier on Monday.

“One state can chart the course, not just for the next four years, but for the next generation.”

Georgia’s January elections, necessary because no Senate candidates won a majority of the general election votes, have been unique for many reasons, not least because the contenders essentially ran as teams, even campaigning together sometimes.

One contest features Democrat Raphael Warnock, the senior pastor of the Atlanta church where murdered civil rights leader Martin Luther King Jr grew up and preached.

The 51-year-old black man was raised in public housing and spent most of his adult life preaching in Baptist churches.

He is facing Republican senator Kelly Loeffler, a 50-year-old former businesswoman who was appointed to the Senate less than a year ago by the state’s Republican governor.

She is only the second woman to represent Georgia in the Senate, although race has emerged as a campaign focus far more than gender. Ms Loeffler and her allies have seized on some snippets of Mr Warnock’s sermons to cast him as extreme.

The other election pits 71-year-old former business executive David Perdue, who held the Senate seat until his term officially expired on Sunday, against Democrat Jon Ossoff, a former congressional aide and journalist.

At 33 years old, Mr Ossoff would be the Senate’s youngest member if elected. The fresh-faced Democrat rose to national prominence in 2017 when he launched an unsuccessful House special election bid.

Despite fears among some Republicans that Mr Trump’s baseless claims of voter fraud could depress turnout, the party’s two candidates have pledged loyalty to the president.

The results also will help demonstrate whether the sweeping political coalition that fuelled Mr Biden’s victory was an anti-Trump anomaly or part of a new landscape.

The Democrat won Georgia’s 16 electoral votes by about 12,000 ballots out of five million cast in November.

Democratic success is likely to depend on driving a huge turnout of African Americans, young voters, college-educated voters and women, all groups that helped Mr Biden become the first Democratic presidential candidate since 1992 to win Georgia.

Republicans have been focused on energising their own base of white men and voters beyond the core of metro Atlanta.