What to watch: Senator Jeff Flake, a Republican who has been a persistent critic of President Trump, is not seeking re-election. The Republican primary field to replace Mr. Flake includes Representative Martha McSally, as well as candidates who have highlighted their loyalty to Mr. Trump: Kelli Ward, a former state senator, and Joe Arpaio, the former sheriff whose conviction for criminal contempt was pardoned by the president last year. On the Democratic side, Representative Kyrsten Sinema, a centrist Democrat, will face off against Deedra Abboud, a progressive. Ms. Sinema, a lawyer, has a significant lead over Ms. Abboud in both fund-raising and the polls. Read more

— LIAM STACK

Read more about today’s most important races.