Donald Trump has decided to intervene directly in the runoff race for the Senate in Alabama. Incumbent Luther Strange, endorsed by Trump, is trailing Judge Roy Moore by 14 points in a recent poll.  Senate Republicans have been begging the president to make a campaign appearance for Strange in the state and yesterday, Trump agreed to hold a rally in Huntsville next Saturday.

I will be in Huntsville, Alabama, on Saturday night to support Luther Strange for Senate. "Big Luther" is a great guy who gets things done!

— Donald J. Trump (@realDonaldTrump) September 16, 2017

Moore has been endorsed by Trump's former chief of staff, Steve Bannon, who sees the race as a test case to challenge other GOP incumbent Senators in 2019.

Politico:

Trump’s unexpected move sets the stage for a showdown between the president and his recently departed chief strategist, Steve Bannon, who is all-in for Moore. Bannon has cast the Alabama race as an-important clash between grass-roots conservatives and the Washington establishment — and a test for whether other incumbent senators can be successfully challenged by insurgents in 2018.

In response, Senate Majority Leader Mitch McConnell and other incumbent senators — including Strange himself — have leaned on the president for more help.

Strange spoke several times with Trump by phone last week and asked him to visit before the election. In one of the calls, Strange told the president that he wanted him to come to Alabama but understood that Trump was focused on a pair of devastating hurricanes, according to three people briefed on the discussion. During the 30-minute conversation, Trump told Strange he supported him but was unsure what he could do.

Strange also pitched Vice President Mike Pence. During a recent conversation, the senator gave Pence an update on how the race was going and contended public polling numbers showing him behind shouldn’t be taken seriously, said two people familiar with the discussion. Strange said he’d be appreciative of anything the White House could do. But there was still no commitment.

“The president is extremely popular here. His approval numbers are in the mid-80s among Republicans,” said Blake Harris, a Republican strategist in the state. “Even more, he's got a record of drawing huge crowds in this state — so a visit could definitely make a difference in what is predicted to be a pretty low turnout election.”

Strange’s Senate colleagues got in on the push, too. Tennessee Sen. Bob Corker, who is up for reelection in 2018 and faces the prospect of a primary challenge, spoke extensively with Trump on Friday and urged him to get involved, according to two people familiar with the conversation. And this week, Sen. Jim Inhofe (R-Okla.) told White House Chief of Staff John Kelly in a phone call that he’d love to see the president head to Alabama for Strange, said two people briefed on the discussion. Kelly told Inhofe that no decision had been made.

Moore would be a weaker general election candidate than Strange, but in Alabama, it hardly matters. The state is one of the most Republican in the nation.

But Bannon is bound and determined to destroy establishment Republicans on the Hill. Moore's healthy lead over Strange in the polls show that there is support among rank and file Republicans for Bannon's effort to shake up the party.

Each Senate race is different and finding the right primary challenger to run against incumbent Republicans will be difficult. Moore started with fantastic name recognition and ready access to cash. He is also effectively using social media. Can Bannon duplicate this scenario in other states where a GOP incumbent might be vulnerable? 

It's a tall order, but the Moore/Strange race shows that grassroots Republicans may be ready to fully embrace the anti-establishment message.