It reads better with a couple of small changes in the punctuation:
'Sen. Steve Daines, Rep. Greg Gianforte, and State Auditor Matt Rosendale. Who is challenging Tester this November?
Newspapers haven't been worth reading since they got rid of the copy editors.
...who is challenging Tester this November....
...
Pretty sure the antecedent for “who” is Rosendale.