Helfrich’s resignation comes during a year of considerable turnover in leadership for both parties. Republicans will enter next year’s session with new leaders in both chambers, while Democrats will have a new House speaker and majority leader and a new Senate majority leader.

After all the races were called, Republicans held a 53-47 advantage that gave them control of the Senate for the first time in four years.

Republicans are getting closer to winning the House, although several races in California and Western states have yet to be called. That could lead to total GOP control of Congress.

President-elect Donald Trump's Republican Party will control both houses of Congress when he takes office in January, Decision Desk HQ projected on Monday, enabling him to push an agenda of slashing taxes and shrinking the federal government.