Sen. Luther Strange trailed former state Supreme Court Justice Roy Moore, both Republicans, by 1 percentage point in a closely watched special Senate election in Alabama.

The upstart Moore led Strange, appointed when Jeff Sessions resigned to become U.S. attorney general, 41 percent to 40 percent. The special election is Sept. 26.

The survey was produced by Republican pollster Jan van Lohuizen Sept. 9-10 for Senate Leadership Fund, a super PAC tied to Senate Majority Leader Mitch McConnell, R-Ky., that is supporting Strange. President Trump also is backing the senator.

"In just over a month's time, the gap between Moore and Strange has shrunk, from 6.1 points (Moore's margin over Strange in the August 15th primary election) to a virtual dead heat," van Lohuizen said in a memo detailing the poll's findings.

Here is the methodology of the poll as laid out in the memo:

"The survey was conducted by telephone on 9/9 and 9/10/2017. We interviewed 604 GOP primary voters; the voters were selected randomly from a voter file consisting of people with a history of voting in Republican primaries. The margin of error associated with these findings is +/- 4 percent."

The RealClearPolitics average of recent polls in this race show Moore with a more substantial lead, 44.4 percent to 33.4 percent.

A Moore victory would be a blow to McConnell and could embolden anti-establishment candidates to challenge in Republican incumbents in primaries in 2018. Steve Bannon, head of Breitbart News and former White House adviser to Trump, is vowing to target McConnell's allies in nominating contests next year.

A group he is affiliated with, Great America Alliance, has purchased $26,000 television advertising time for Saturday, to run on ESPN 2 during the University of Alabama football game, according to media buying sources. The group this week unveiled a new hard-hitting spot against Strange that left some questioning whether it crossed the line for what kinds of ads political nonprofits are legally permitted to run.

Meanwhile, the Senate Leadership Fund's latest poll showed Trump maintaining a strong 83 percent favorable rating among GOP primary voters. That should be an advantage for Strange, especially if the survey is correct and Moore's favorable ratings are slipping.

"We saw a substantial change in the image ratings for Roy Moore. Thirty-nine percent now have an unfavorable impression of Moore, an increase of 8 percent compared to 2 weeks ago, while favorable impressions declined by 5 percent (from 59 percent to 54 percent,)" van Lohuizen wrote in the memo.