The U.S. Chamber of Commerce on Friday stepped up its involvement for Sen. Luther Strange in the Sept. 26 special Senate election in Alabama, as a campaign with broad ramifications for 2018 entered the final stretch.

The chamber is dropping a direct mail piece across the state featuring an endorsement from Sen. Richard Shelby, R-Ala. This is Shelby's first foray into the special election campaign. The chamber also is hitting the television airwaves in Birmingham and Huntsville, beginning Saturday during the Auburn University and University of Alabama college football games. The organization was slated to spend $138,000 on television through Election Day.

Strange, appointed to the Senate in January to succeed Jeff Sessions when he retired to become U.S. attorney general, is locked in a close Republican primary race with upstart candidate Roy Moore, former Chief Justice of the Alabama Supreme Court. His upset victory could encourage a spate of primary challenges against Republican Senate incumbents next year that attract the resources they need to be competitive.

Strange was endorsed by President Trump, perhaps the ultimate outsider. But as the incumbent with the strong support of Senate Majority Leader Mitch McConnell, R-Ky., and groups linked to him, Strange has suffered from the anti-establishment fever coursing through GOP primary voters in Alabama, allowing Moore to claim outsider mantle and accrue endorsements from conservative activists at odds with Republican leaders in Congress.

"I'm taking the fight directly to the establishment," Moore wrote in an email fundraising appeal. Asked to explain, given that claim, why Trump backed Strange over him, Moore told conservative talk show host Laura Ingraham that Jared Kushner, the president's son and law and close advisor, led him astray. "It could be Kushner," he said.

Moore was set to spend $533,000 on statewide television advertising during the final four weeks of the campaign. Great America Alliance, a political nonprofit linked to Steve Bannon, head of Breitbart News and former White House advisor to Trump, was to spend $27,000 to advertise on ESPN2 on Saturday during the Alabama game.

There was some additional spending for Moore by outside groups, but overall he and his allies were swamped by Strange and the organizations backing his campaign. Over the final month of the campaign, the Strange campaign had booked $758,000 of television time; Senate Leadership Fund, the super PAC aligned with McConnell, had purchased $3.3 million; and the National Rifle Association's political arm had invested $525,000.

Strange also has an operational advantage, running a modern campaign, compared to Moore, who is relying on his network churches and families who home school their children to get out the vote.

In a memorandum to supporters detailing Strange's get-out-the-vote machine, the senator's campaign revealed that since round one of the special Senate GOP primary on Aug. 15, its permant volunteer army of about 110-120 individuals has knocked on more than 45,000 doors across the state, especially focusing on the targeted counties of Jefferson, Shelby, Madison, Mobile and Baldwin.

Additionally, the Strange campaign has logged nearly 48,000 phone calls to potential voters; signed more than 11,000 new supporters to its grassroots network; distributed 4,200 signs; visited 48 small businesses and attended four college football games. In an interview, the Strange campaign expressed confidence in the outcome.

Alabama Republicans, Strange campaign advisor Kristin Davison said, "see that Sen. Strange is a reliable Trump supporter in the Senate and fights for them on the issues they care about."