Donald Trump's Republicans were projected to pick up another U.S. Senate seat on Thursday and appeared to be retaining their hold on the House of Representatives, putting them on track to control both chambers of Congress next year.

Ticket-splitting used to be common and the swing-state down-ballot results show split tickets still can determine the outcome of key races.

That margin could climb as high as 55 seats if Republicans win close races in Arizona and Nevada, which remain uncalled. In the battle for the House, Republicans were edging closer to victory. Republicans had secured 211 seats,

Political control of the Arizona Senate was on the ballot Tuesday. Counting by the end of the week has revealed a trendline that favors Republicans retaining control of the 30-member chamber, although hundreds of thousands of ballots remain to be counted.