In the big-money Republican primary for senator from Alabama, the U.S. Chamber of Commerce is endorsing incumbent Sen. Luther Strange over Alabama Chief Justice Roy Moore, cementing what has already become a high-profile race pitting Republican leadership against an insurgent social conservative.

President Trump endorsed Strange in the primary's first round — not because he especially liked Strange, but because Strange's challenger, Mo Brooks, had said a ton of negative things about Trump during the election campaign.

• The president had not formed an opinion about Roy Moore, according to sources, and the purpose of the Strange endorsement was to block Brooks. Trump has been much quieter since the race moved into the runoff phase.
• Advisers have shown Trump the polls that have Moore well ahead of Strange. On Aug. 16, the president appeared to hedge his bets in a tweet: "Congratulation [sic] to Roy Moore and Luther Strange for being the final two and heading into a September runoff in Alabama. Exciting race!"

Why it matters: Moore, a favorite of social conservatives, has received enthusiastic support from Steve Bannon and Breitbart News in a race that's testing the strengths of the party's right-wing and more moderate camps.

• Much of the energy behind Moore is simply anti-McConnell, anti-establishment. Moore is the beneficiary of this widespread frustration and he's never had a track record on trade or immigration — the core issues of the nationalist populist base. In fact, Moore embarrassed himself in a radio interview because he didn't even know what DACA was when asked about it.
• Those same anti-establishment forces backing Moore will inevitably use the Chamber's support to attack Strange, whom they're already branding as a D.C. insider / ex-lobbyist. The Chamber has substantial resources, however, and the group's support is a net plus given the insurgent-versus-establishment dynamics were already well set in this race.
• Backing Moore: Bannon, Breitbart News, House Freedom Caucus Chairman Mark Meadows and former Arkansas Gov. Mike Huckabee. Backing Strange: The business community and Senate Majority Leader Mitch McConnell.