Who Controls the Senate in a 50-50 Tie? 

Article I, Section 3 of the Constitution specifies that The Vice President is the President of the Senate. The Vice President has no vote unless to break a tie in some matter before the Senate. 

Therefore, if the election leads to a 50-50 party split in the Senate, the Vice-President's vote will give the majority to his or her party. 

Currently, the U.S Senate is controlled by the Republicans. As of January, 2018, they control 51 Senate seats, Democrats 47, with 2 independents caucusing as Democrats (effectively giving them 49 seats). In 2018, 35 Senate seats are up for election, 26 held by Democrats, 9 by Republicans. Since Republican Vice-President Mike Pence will break any ties, Democrats must gain at least 2 seats, for a minimum of 51 total, to take control in early January, 2019.