Republicans held on to key Senate seats in Indiana and Florida on Tuesday as they sought to hold off the Democrats' bid to retake the Senate majority.
But Democrats grabbed a Republican-held seat in Illinois in a race that has been described as a "coin flip" between the two parties.
The main focus of US and world media has been on the brutal race for the White House between the Republican nominee Donald Trump and Democratic candidate Hillary Clinton. Historically, Presidents have always struggled to pass their agenda while contending with an opposition-run House and Senate.
Republicans hold a majority of 54 to 46 in the Senate, which means Democrats need to pick up just four seats to take the majority if Ms Clinton wins the White House.
Some observers have suggested the possibility of a hung Senate providing the Vice President, either Republican Mike Pence or Democrat Tim Kaine, with the deciding vote.