Control of the US Senate remains in flux on Friday with critical vote counts ongoing in Nevada and Arizona as Republicans appear on the precipice of claiming the House of Representatives.

Democrats have added to their narrow lead in the battle to defend Arizona Sen. Mark Kelly's seat from Trump-endorsed Republican challenger Blake Masters. Kelly is ahead by some 5.6 percentage points, according to The Associated Press.

Critically, officials are expected to begin announcing results on Friday from a batch of roughly 300,000 mail ballots that were returned on Election Day. The ballots could swing the race one way or the other after they are tabulated.

In Nevada, Republican candidate Adam Laxalt is just 1 percentage point ahead of incumbent Sen. Catherine Cortez Masto as 10% of the vote remains outstanding. Counting remains ongoing.

Senate run-offs are planned in Georgia and Alaska, though the run-off in Alaska is slated to pit two Republicans against one another, meaning regardless of who wins, the GOP will lay claim to the state's Senate seat.

In Georgia, Democratic Sen. Raphael Warnock and Republican challenger Herschel Walker are headed for a Dec. 6 runoff that could provide either party with the seat it needs to claim Senate control.

Republicans currently have control of 49 of the Senate's 100 seats, compared to 48 for Democrats.

Democrats need 50 seats to hold a Senate majority thanks to Vice President Kamala Harris, while Republicans would need 51 since their party does not currently hold the White House.

An American vice president serves as president of the Senate and is able to cast tie-breaking votes. Senate tradition has held that control of the chamber in an evenly-split body goes to the party that controls the White House.

In the House, Republicans have steadily added to their total and are now just seven short of being able to claim a majority. Currently, Republicans hold 211 seats compared to 192 for Democrats. Many of the outstanding races are in the more liberal-leaning West, particularly California. Some 27 races have yet to be called.

Either party needs to win 218 seats for a majority.

If Republicans win the House, President Joe Biden’s legislative agenda will likely be almost certainly stopped in its tracks, facing an uncooperative Republican-dominated body.

If Republicans also take control of the Senate, Republicans could also block or slow down many of Biden’s appointments to key administration positions, as well as federal judges, who enjoy lifetime appointments, helping to shape public life for decades.