A poll released on Friday that shows Democrat Dan McCready leading Republican Dan Bishop by four points in the September 10 special election in North Carolina’s Ninth Congressional District apparently oversampled Democrats.

“The poll, conducted jointly by Republican polling firm Harper Polling and Clarity Campaign Labs, a Democrat polling firm, surveyed 551 likely special election voters for Inside Elections magazine between August 26 and August 28, showing McCready with 46 percent to Bishop’s 42 percent. McCready’s 4-point lead is within the poll’s 4.2 percent margin of error,” Breitbart News reported on Saturday.

“According to the weighted toplines, the party breakdown of poll respondents was Democrat plus 10, with 42 percent of respondents identifying as Democrat and 32 percent identifying as Republican. In contrast, the Cook Partisan Voting Index (PVI) for North Carolina’s Ninth Congressional District is Republican plus 8,” Breitbart News noted.

The sampling methodology used by Harper Polling/Clarity Campaign Labs in their poll showing Democrat McCready with a four point lead differs dramatically from the sampling methodology used by the only other poll of the race that has been released to the public to date, a May 2019 poll of the Ninth Congressional District special election conducted by JMC Analytics, which showed Republican Dan Bishop with a four point lead over over McCready, 46 percent to 42 percent.

It is worth noting that the results of both polls–the August Harper Polling/Clarity Campaign Labs Poll and the May JMC Analytics Poll–showed the race between Republican Dan Bishop and Democrat Dan McCready is a statistical tie. The margin of error in the August Harper Polling/Clarity Campaign Labs Poll is 4.2 percent, making Democrat McCready’s four point lead within the polls margin of error. Similarly, the margin of error in the May JMC Analytics Poll is 5.2 percent, making Republican Bishop’s four point lead within the margin of error.

The party identification breakdown of respondents in the May JMC Analytics Poll was Republicans plus 11 (52 percent Republican and 41 percent Democrat), a marked contrast to the party identification breakdowns of respondents in the August Harper Polling/Clarity Campaign Labs Poll, which was Democrats plus 10 (32 percent Republican, 42 percent Democrat).

The party identification breakdown of respondents included in the May JMC Analytics Poll was just three points more weighted towards Republicans than the Cook PVI for the district of Republican plus 8.

In contrast, the party identification breakdown of respondents included in the August Harper Polling/Clarity Campaign Labs Poll was whopping 18 points more weighted to Democrats than the Cook PVI for the district of Republican plus 8.

“First introduced in 1997, the Cook PVI measures how each district performs at the presidential level compared to the nation as a whole,” the Cook Political Report notes.

What these means for North Carolina’s Ninth Congressional District is that in the most recent presidential elections, the Republican candidate for president has, on average, performed eight points better in this district than in the national popular vote. That Republican differential was even greater in the 2016 presidential election, when President Donald Trump won the district by 12 points over Hillary Clinton, which is 14 points better than he performed in the national popular vote, which Clinton won by two points, 48 percent to 46 percent. While Clinton won the popular vote, she lost the presidency because Donald Trump received 304 electoral college votes to her 227 electoral college votes, with seven “faithless electors” casting votes for other candidates.

North Carolina’s Ninth Congressional District stretches from its western boundary in Charlotte to its eastern boundary in the middle of Bladen County about 60 miles from the Atlantic Ocean. Its southern boundary is the state border with South Carolina.

The district includes all or part of nine different counties. Its demographic makeup by race is 79 percent White, 14 percent Black, and 4 percent Asian. In addition, 7 percent of the population is ethnically identified as Hispanic (presumably a subset of the other racial identities).

Until January 2019, the district was represented in the U.S. House of Representatives by Rep. Robert Pittenger (R-NC), who defeated Democrat Christian Cano in the 2016 election by a 58 percent to 42 percent margin.

Dr. Mark Harris defeated Pittenger in the May 2018 Republican primary, and was thought to the winner by 905 votes of the November 2018 midterm election against Democrat Dan McCready until the North Carolina Board of Elections refused to certify the election results when allegations of election ballot improprieties were brought to their attention. In January, the NCBE declared a new special election was to be held on September 10 to determine who would represent the district in the U.S. House of Representatives, and Harris chose not to enter that race.

Pittenger was first elected to represent the district in 2012, when he defeated his Democrat opponent Jennifer Roberts by a six point margin, 52 percent to 46 percent. His hold on the district was considered so strong that Democrats did not even have a candidate against him in 2014, and he easily defeated a write-in candidate with 93 percent of the vote.

But by early 2016, conservatives in the district tired of Pittenger’s moderate voting record, and he narrowly defeated Harris in a May 2016 Republican primary, 36 percent to 35 percent. Harris challenged Pittenger in the primary again in May 2018, and this time succeeded in defeating him, this time by a 48 percent to 46 percent margin.

The district’s voting behavior has been remarkably consistent in the two most recent presidential elections. In 2016, Donald Trump defeated Hillary Clinton by 12 points, 55 percent to 43 percent, even though he lost the national popular vote by two points, 48 percent to 46 percent. Similary, in 2012, Republican nominee Mitt Romney beat President Obama in the district by that same 12 point margin, 55 percent to 43 percent.

In the analysis accompanying the release of August Harper Polling/Clarity Campaign Labs Poll, Harper Polling, the Republican polling firm in this bipartisan polling effort, explained that they believed historical voting behavior does not really apply to this September 10 special election in North Carolina’s Ninth Congressional District:

While each special election brings a unique set of circumstances—the September 10th election in North Carolina’s 9th Congressional District is so “special” that it can’t even be called that. This historic “redo” election is the spawn of voter fraud committed on behalf of the 2018 Republican nominee. Whether voters ultimately delineate between Dan Bishop and those GOPers responsible for the fraud is unknown. As context for the election, this dynamic must be remembered amidst the emphasis on the district’s electoral performance in prior Presidential elections. Those numbers are not neatly applicable to this scenario. (emphasis added)

Clarity Campaign Labs, the Democrat polling firm in this bipartisan polling effort, offered this perspective on the poll showing McCready in the lead:

Overall, these results give Democrats lots of reasons for optimism. McCready has a small but real lead, built largely on college-educated voters in Mecklenburg County. He leads among voters who have decided, and undecided voters are leaning his way two-to-one. These are the same types of suburban voters who drove Democratic gains across the country in 2018, and it’s a great sign that they’re still enthusiastic about Democratic House candidates. The biggest caution is that winning in this challenging district will depend on keeping that enthusiasm edge. McCready leads by 5% among people who say they’re very excited about voting, but Bishop leads among voters who say they’re only a little excited about turning out. We’ve seen this pattern in special elections in the South, especially in GA-06 in 2017: highly motivated college-educated suburban Democrats and less-motivated Republicans. We know that Republican efforts in the last week will be aimed at driving up turnout outside Mecklenburg County, and in a district this tough that could make the difference. President Trump breaks even in this district – it’s a drop from 2016, when he carried the seat by 12%, but he’s still more popular here than in the state or nation as a whole. Senator Mitch McConnell, however, is underwater at -23%. Voters in this district, like voters across the country, like Congressional Republicans a lot less than they like the President, and continuing to make the case that Dan Bishop would be more of the same in DC will be key to finishing the race strongly.

Breitbart News contacted both Harper Polling and Clarity Campaign Labs to comment on the apparent over sampling of Democrats contained in their poll, but has not yet received a response.