This is the first time YouGov has conducted a survey pitting Mr. Roberts against Mr. Orman. A tied race in Kansas is somewhat good news for Republicans because other recent polls have shown Mr. Orman with a modest lead. On the other hand, the good news for Mr. Orman was that 19 percent of Democrats were undecided, compared with just 9 percent of Republicans. Recent polls by USA Today/Suffolk University and Fox News did not find a disproportionate share of Democrats still undecided, and that might explain part of why Mr. Orman is faring worse here than in other polls.

The Roberts campaign began airing advertisements in mid-September, ending Mr. Orman’s uncontested dominance of the airwaves. But the data suggests that these efforts haven’t improved Mr. Roberts’s standing. Thirty-two percent of voters said that recent advertisements had made them more likely to support Mr. Orman, compared with 25 percent for Mr. Roberts. Similarly, 25 percent of voters said that what they’d learned over the last few weeks led them to think worse of Mr. Roberts, compared with just 12 percent who think better of him.

Outside Kansas, the results are broadly similar to the last wave of YouGov data.

Some of the shifts since the last wave of results, in September, reflect the changing opinions of the 65 percent of panelists who were interviewed last time. But more than 90 percent of past respondents in the nine competitive states other than Kansas supported the candidate they backed in the last poll. The shifts among the 10 percent who did change their answers usually went in and out of the undecided column, and most of those shifts canceled out. Other changes resulted from the addition of 30,687 new panelists; the reweighting of the new sample; and the changes in whether respondents were considered “likely” voters.

The new data offers some encouraging news for Democrats in Colorado and Iowa, two states where other recent polls have turned toward the Republicans. The YouGov data shows Republicans gaining about a percentage point among past respondents since the last wave. These modest shifts were not enough to give Republicans the lead in either, but both races are very close. The Democrat Bruce Braley’s one-point lead in Iowa over Joni Ernst holds only by the margin of rounding: He leads by 43.8 to 43.4, essentially a dead heat.