Before Tuesday’s elections, the Republican and Democratic caucuses were evenly divided in the Senate, with vice president Kamala Harris able to break ties when necessary.
Republicans presently hold a majority in three swing states, so if they win one more, they will have a majority.
Republican Blake Masters is trailing Democrat Mark Kelly in Arizona, although just 63% of the ballots have been counted.
Georgia’s election is too close to call, and if neither candidate receives 50% of the vote, a runoff may be held on December 6.
The incumbents have won Ohio, New Hampshire, and Colorado’s projected competitive senate races.