McSally had led by 2 points in February, and by 1 point in May. She is now trailing by 5 points (Kelly 46% to McSally 41%).
This is McSally's second attempt to be elected as Arizona's senator. She lost to Kirsten Sinema in the last election. She was then appointed to replace John McCain after his death -- making her the non-elected incumbent. If she can't hold on to the seat, her political career is probably over.
This is a real possibility for Democrats to grab a Republican-held Senate seat. Mark Kelly is popular, as is his wife "Gabby" Giffords (who was a U.S. Representative from Arizona before being shot in the head at a political rally).
Republicans are blaming Trump's poor job approval numbers in Arizona for McSally trailing. That probably didn't help, but I think the Democrats just have the better candidate in this race.
The chart above reflects to results of the new OH Predictive Insights Poll -- done on August 13th and 14th of 600 likely Arizona voters, and has a 4 point margin of error.