Democrats and Republicans were locked in tight US Senate races in Georgia as final votes were counted in a showdown that will decide whether President-elect Joe Biden enjoys control of Congress or faces stiff Republican opposition to his reform plans.

Despite the close margins, the Democratic challenger in one of the contests, the Rev. Raphael Warnock, told supporters that he would be going to the Senate, though no major news outlet had projected a winner and his rival, Republican Senator Kelly Loeffler, said she still had a path to victory.

The close margins in that race and the contest between Republican Senator David Perdue and hi Democratic challenger, documentary filmmaker Jon Ossoff, meant the winners might not be clear until at least Wednesday morning.

With 97% reporting, Warnock was ahead of Loeffler by less than a percentage point and Ossoff had pulled into a dead heat with Perdue, according to Edison Research.

The critical races drew an estimated 4.5 million voters - a record for a runoff - along with nearly half a billion dollars in advertising spending since Nov. 3 and Monday visits by Republican President Donald Trump and President-elect Joe Biden.

Georgia Secretary of State Brad Raffensperger said election officials would take a break overnight but resume counting on Wednesday morning. "Hopefully by noon we'll have a better idea where we are," he said on CNN.

Warnock voiced confidence in a video message to supporters.

"I am honored by the faith that you have shown in me, and I promise you this tonight, I am going to the Senate to work for all of Georgia, no matter who you cast your vote for in this election," he said.

But Loeffler said she still believed she would win.

"We have a path to victory and we're staying on it," Loeffler told supporters in Atlanta. "It's worth it for this election to last into tomorrow. We're going to make sure every vote is counted."

Most of the votes remaining to be counted were in counties Biden won in November, with roughly 30,000 to go in DeKalb and Newton counties near Atlanta, according to Edison Research estimates.

Democrats must win both contests to take control of the Senate. A double Democratic win would create a 50-50 split in the Senate and give Vice President-elect Kamala Harris the tie-breaking vote after she and Biden take office on Jan. 20. The party already has a narrow majority in the House of Representatives.

If Republicans hold even one of the two seats, they would effectively wield veto power over Biden's political and judicial appointees as well as many of his legislative initiatives in areas such as economic relief, climate change, healthcare and criminal justice.

No Democrat has won a US Senate race in Georgia in 20 years. The head-to-head runoff elections, a quirk of state law, became necessary when no candidate in either race exceeded 50% of the vote in November.

If elected, Warnock would become Georgia's first Black U.S. senator and Ossoff, at 33, the Senate's youngest member. Perdue is a former Fortune 500 executive who has served one Senate term. Loeffler, one of the wealthiest members of Congress, was appointed a year ago to fill the seat of a retiring senator.

DEMOCRATIC OPTIMISM

Biden's narrow statewide win over Trump in the Nov. 3 election - the first for a Democratic presidential candidate since 1992 - gave the party reason for optimism in a state dominated by Republicans for decades.

In Smyrna, about 16 miles (26 km) northwest of Atlanta, Terry Deuel said he voted Republican to ensure a check on Democratic power.

"The Democrats are going to raise taxes," the 58-year-old handyman said. "And Biden wants to give everyone free money - $2,000 each or something like that for COVID stimulus? Where are we going to get the money?"

Ann Henderson, 46, cast ballots at the same location for Ossoff and Warnock, saying she wanted to break Washington's gridlock by delivering the Senate to Democrats.

"It's the social issues - civil rights, racial equality, voting rights, pandemic response," she said. "If we take it, maybe we can get something done for a change."

U.S. equity market index futures were broadly weaker as the results turned in favor of the Democrats, signaling stocks could open on the soft side on Wednesday morning. The benchmark S&P 500 e-mini futures contract was down 0.6%, while futures tracking the tech-heavy Nasdaq were off by 1.3%.

The campaign's final days were overshadowed by Trump's efforts to subvert the presidential election results.

On Saturday, Trump pressured Georgia Secretary of State Brad Raffensperger, a fellow Republican, on a phone call to "find" enough votes to reverse Biden's victory, falsely claiming massive fraud.

Trump's bid to undo his loss - with some Republicans planning to object to the certification of Biden's win when Congress meets on Wednesday to formally count the presidential vote - have split his party and drawn condemnation from critics who accuse him of undermining democracy.

At a rally in Georgia on Monday night, Trump again declared the November vote "rigged," an assertion some Republicans worried would dissuade his supporters from voting on Tuesday.

His attacks appear to have undermined public confidence in the electoral system. Edison's exit poll found more than seven in 10 were very or somewhat confident their votes would be counted accurately, down from 85% who said the same in a Nov. 3 exit poll.

Reuters