Democratic Rep. Kyrsten Sinema (AZ-09) announced she will run for U.S. Senate in 2018. This gives the party a high-profile candidate in their efforts to unseat incumbent Republican Jeff Flake. While Arizona hasn't had a Democratic Senator in over 20 years, next year's race is expected to be highly competitive. The Democratic nominee may even be favored if Flake loses a primary to former State Senator Kelli Ward. A recent poll by GBA Strategies showed him losing to Ward by a 58-31% margin.
Flake's national visibility was raised earlier this summer with the publication of his book "Conscience of a Conservative", which harshly critiqued President Trump and his own party. Needless to say, this hasn't helped his standing with Arizona Republicans. The GBA Strategies poll gave Flake just a 25% approval rating among Republican primary voters in the state.
In her first run at U.S. Senate, Ward lost the Republican primary to Sen. John McCain in 2016.
Sinema becomes the 26th House member to pass on running for reelection to the House in 2018. The next such announcement is likely to come from Tennessee, where Republican Marsha Blackburn is likely to run for the seat being vacated by Sen. Bob Corker.