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. They have 52 Senate seats, Democrats 46, with 2 independents caucusing as Democrats (effectively giving them 48 seats). In 2018, 33 Senate seats are up for election, 25 held by Democrats, 8 by Republicans. Since Republican Vice-President Mike Pence will break any ties, Democrats must gain at least 3 seats, for a minimum of 51 total, to take control in early January, 2019.