“So far, incumbent senators of both parties who have run for renomination are a perfect 18 for 18 in primaries,” said Kyle Kondik, of the University of Virginia’s Center for Politics. “Roberts and Alexander appear to be favorites, even though tea party groups in both states back the challengers. Of the two, Mr. Roberts is probably more vulnerable to an upset: He’s run a subpar campaign and has really suffered from questions about his residency in Kansas.”