While 2018 is a long way away, Alabama is having a special election to fill Jeff Sessions' seat this very year that could potentially tip the Senate's balance to a slim 51-49 majority for the GOP. Primaries were held today before election in December and the ongoing results can be found here: https://www.nytimes.com/elections/results/alabama-senate-special-election-primary
EDIT: Roy Moore and Luther Strange will be in a runoff election on Spetember 26th as neither candidate received more than 50% of the vote. The winner will go on to face Doug Jones in December for the seat.
EDIT: Roy Moore and Luther Strange will be in a runoff election on Spetember 26th as neither candidate received more than 50% of the vote. The winner will go on to face Doug Jones in December for the seat.