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, 2019, they control 53 Senate seats, Democrats 45, with 2 independents caucusing as Democrats (effectively giving them 47 seats). In 2020, 35 Senate seats are up for election, 23 held by Republicans, 12 by Democrats. As the Vice-President will break any ties, Democrats would need to gain 3 seats and win the presidency to take control in a 50-50 Senate as of January, 2021.