WASHINGTON (AA) – The question of who will claim control of either chamber of Congress remains very much open Wednesday, one day after millions of voters cast ballots nationwide in the US’ midterm elections.

Currently, Democrats have flipped a key Senate seat in the state of Pennsylvania as they seek to claim an outright majority in the chamber, but with four races yet to be called by The Associated Press it is anyone’s guess as to which party will win the majority.

The Pennsylvania seat had been tightly contested between victorious Lieutenant Governor John Fetterman and Republican celebrity doctor Mehmet Oz, but Fetterman claimed victory early Wednesday. Oz conceded hours later.

Critical races in Arizona, Georgia, Nevada and Wisconsin have yet to be called.

Democrats need 50 seats to claim a 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.

It is unclear when the remaining races will be called, and it is possible the Republican and Democratic candidates in Georgia could head to a run-off if neither is able to pass a 50% threshold. A December run-off pitting Senator Raphael Warnock against Republican challenger Hershel Walker is becoming increasingly likely in the southern state.

With more than 95% of the vote counted, Warnock holds a razor-thin edge against Walker that stands at less than 1%. The senator currently holds about 49.4% of the vote to Walker’s 48.5%. A Libertarian candidate holds just over 2%.

Full results could take days to process in outstanding states.

Republicans have been forecast to win 199 of the 435 seats in contest in the House of Representatives. Democrats, meanwhile, have won 174, according to The AP. Either party needs to claim 218 seats to win a majority.

In addition to all 435 House seats, there are 35 seats in the Senate that are up for grabs in the 100-member chamber as well. The vast majority of contested Senate seats, 21, were held by Republicans this election cycle. One-third of all seats in the chamber go up for election every six years, while all House seats are contested every two years.

If Republicans win the House, as is likely, President Joe Biden’s legislative agenda will 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.