Fox News polls released on Wednesday show Joe Biden gaining momentum in the key battleground states of Wisconsin, Arizona, and North Carolina, but those results appear to reflect significant oversampling of Democrats, particularly in Arizona and North Carolina.

In Arizona, for instance, the Fox News poll shows the former vice president with a nine point lead over the president among likely voters, 49 percent to 40 percent.

In Wisconsin the Fox News poll shows former Vice President Biden with an eight point lead over President Trump among likely voters, 50 percent to 42 percent.

And in North Carolina, the Fox News poll shows Biden leading Trump by four points, 50 percent to 46 percent.

Three other recent major polls conducted in August in Wisconsin showed an average lead for Biden of just 2.7 points. In Arizona, two other recent major polls conducted in August showed an average lead for Biden of three points. In North Carolina, three other recent major polls conducted in August showed an average lead for Trump of one point.

Fox News polls have been criticized for oversampling Democrats, as Brian Joondeph wrote in this article published at Rasmussen Reports in July:

The Fox News poll [of national voters conducted in July] oversampled Democrats by four points, and in the “who would you vote for today,” Biden won by an eight-point margin. But the poll was split evenly on whether Trump would be reelected.

Breitbart News reported about a poll Fox News conducted the month before the December 2017 special U.S. Senate election in Alabama. (On the eve of the special election, the Fox News poll showed Democrat Doug Jones with a ten point lead over Republican Roy Moore. Jones won the election by a margin of 1.5 percent, which is 8.5 percent less than the margin indicated in that Fox News poll.)

Oversampling Democrats appears to be a methodological problem that may apply to this current round of Fox News polls in battleground states.

In Arizona, for instance, where the Fox News poll of 732 likely voters was conducted between August 29 and September 1 (with a 3.5 percent margin of error), the political identification of respondents skewed heavily Democrat.

Forty-six percent of respondents identified as Democrat, compared to 43 percent who identified as Republicans, with 11 percent undecided (using the Partisan Voting Index developed by the Cook Political Report, the sample was Democrats plus three, or D +3). This represented a significant increase in Democrat respondents from the October 2018 poll of Arizona voters conducted by Fox News, where 40 percent of respondents identified as Democrats, 50 percent identified as Republicans, and ten percent identified as independents, which, using the PVI method, was a sample that was R +10.

Compared to the October 2018 Arizona poll, Fox News has included six percent more Democrats and seven percent fewer Republicans, a net change of 13 percent towards Democrats and away from Republicans, in its sample of poll respondents.

The most current PVI for the state of Arizona as listed on the Cook Political Report website is R +5. The average PVI of the state’s nine congressional districts (which also includes 2018 results) is R +2, according to this summary of House District PVIs provided by the Cook Political Report.

Notably, the Fox News poll of Arizona respondent sample was at least five points more Democrat, and as much as eight points more Democrat (D +3 compared to R +2 or R +5) than the Cook Political Report PVIs for the state would suggest.

In Wisconsin, the Fox News poll of 801 likely voters (with a margin of error of 3.5 percent) was conducted between August 29 and September 1 and consists of a sample of respondents that is 44 percent Democrat, 42 percent Republican, and 15 percent independent, or D +2.

The most current PVI for the state of Wisconsin, as listed on the Cook Political Report website is “even.” The average PVI of the eight congressional districts in Wisconsin in 2020 is D +1, according to the Cook Political Report.

The Wisconsin Fox News poll respondent sample at D +2 was only one to two points more Democrat than the state’s PVI would suggest.

In North Carolina, the Fox News poll of 722 likely voters (with a margin of error of 3.5 percent) was conducted between August 29 and September 1 and consists of a sample of respondents that is 46 percent Democrat, 45 percent Republican, and nine percent Independent, or D +1.

In contrast, the most current PVI for the state of North Carolina, as listed on the Cook Political Report website is R +3. The average PVI of the 13 congressional districts in North Carolina in 2020 is R +3, according to the Cook Political Report.

The Fox News poll of North Carolina respondents sample was four points more Democrat (D +1 compared to R +3) than the Cook Political Report PVIs for the state would suggest.

The Fox News poll results in North Carolina announced on Wednesday are in dramatic contrast to the East Carolina University poll released on Tuesday, which shows Trump leading Biden in the state by two points, and receiving 13 percent support among black voters in the state, as Breitbart News reported:

President Trump now leads Democratic presidential nominee Joe Biden by two points in the key battleground state of North Carolina, 49 percent to 47 percent, according to a poll released by East Carolina University on Tuesday. . . [the president] leads Biden among white voters by a 25 point margin, 61 percent to 36 percent, while former Vice President Biden leads the president among black voters by a 66 point margin, 79 percent to 13 percent.

…

Twenty-one percent of black male voters support President Trump, while only seven percent of black female voters back the president.

In the head-to-head matchup, 74 percent of black male voters say they intend to vote for Biden, while 21 percent say they intend to vote for Trump. In contrast, 82 percent of black female voters say they intend to vote for Biden, while only seven percent say they intend to vote for Trump.

Black voters make up 20 percent of poll respondents [in North Carolina], while white voters account for 70 percent of poll respondents, a demographic mix comparable to 2016 voting behavior.

Three other recent national polls showed black voter support for Trump at 19 percent or higher, as Breitbart News reported:

An Emerson poll released on Monday found that 19 percent of black voters will vote for the president this November, while a Democracy Institute/Sunday Express poll released on Sunday also found the level of black support for the president at 19 percent, and a Zogby poll released on Monday showed 20 percent of black voters intend to vote for Trump instead of Biden.

The Fox News poll of North Carolina conducted between August 29 and September 1 found that only five percent of black voters in the state say they will vote for Trump, a result that is eight percent lower than the 13 percent black support the president has in North Carolina in the East Carolina University poll.

In an era where many polling firms use polling for the purpose of establishing a political narrative rather than reflecting the true nature of public opinion, details of polling methodology will continue to be carefully scrutinized as readers seek to make sense of polling reports that present widely divergent views of the state of the election.