Former NASA astronaut Mark Kelly (D) announced Tuesday that he is running in the 2020 special election for U.S. Senate in Arizona to complete the term of the late John McCain (R). Kelly became active in politics and gun violence prevention after his wife, former Rep. Gabby Giffords (D), survived an assassination attempt in 2011.

The seat is currently held by Sen. Martha McSally (R), who was appointed in December 2018.

Thirty-three other Senate races will take place in 2020, including at least two for open seats in Tennessee and Kansas where Sen. Lamar Alexander (R) and Pat Roberts (R) announced they were not seeking re-election.

Republicans will be defending 22 seats up for election, compared to Democrats’ 12 seats. That is nearly the reverse of 2018, when Democrats held 26 of the 35 seats up for election.

Two Republicans face re-election in states that President Donald Trump (R) lost in 2018: Sen. Cory Gardner (R) in Colorado, where Trump lost in 2016 by 5 percentage points, and Sen. Susan Collins (R-Maine).

Doug Jones, who won a 2017 special election to become the first Democratic senator elected in Alabama since 1992, is also expected to face a competitive race.

Republicans currently hold a 53-47 majority in the Senate.