The three Democrats join a Senate narrowly split 50-50 between the parties, but giving Democrats the majority with Harris able to cast the tie-breaking vote.

The two Senate runoff elections are leftovers from the November general election, when none of the candidates hit the 50 per cent threshold.

The deadlock became official after Republicans held the Senate seat in Alaska. There, incumbent GOP Sen. Dan Sullivan defeated Al Gross, an independent running as a Democrat, after an onslaught of mail-in ballots delayed counting until Tuesday, a week after Election Day.