The large share of uncommitted voters adds a layer of uncertainty to the race. Remember, we saw the same thing in 2016; Hillary Clinton had a small lead over Donald Trump heading into Election Day, but the unusually high number of undecided voters helped Trump squeak out an Electoral College win.
The uncommitted bloc looks likely to help the GOP in Arizona too. According to the crosstabs of these polls, these Arizonans are disproportionately Republican. For example, in the YouGov poll pitting Sinema against Ward, 92 percent of self-identified Democrats back Sinema, 4 percent aren’t sure and 1 percent want to vote for someone else — but among self-identified Republicans, just 75 percent support Ward, 11 percent aren’t sure and 8 percent say they’ll vote for someone else. In Marist’s Sinema-McSally matchup, 88 percent of Democrats are behind Sinema and just 2 percent are undecided; just 77 percent of Republicans support McSally and 13 percent say they’re undecided.