The plan to elect Luther Strange to the Senate from Alabama was executed with deep pockets, ruthless efficiency and a simple rationale: Donald Trump, the most popular politician in the state, wanted Strange to win.

For weeks, Strange drove home the message in every public appearance. The president blitzed it out on his Twitter account. And outside groups flooded airwaves and mailboxes with more than $9 million in reinforcement: "Big Luther = Trump ally," read one direct mail piece in the final weeks. "If you voted for Donald Trump," read another, "then you CAN'T vote for Roy Moore."

Now, as the dust clears in Alabama, strategists on both sides of the election agree on the reason the strategy didn't work: Trump supporters just didn't believe the president's endorsement made Strange the better candidate to shake up Washington. And they never bought the idea that Moore was against Trump.

"There were a significant number of people who strongly supported the president but voted for Roy Moore," explained Tony Fabrizio, Trump's chief 2016 campaign pollster who also works for the U.S. Chamber of Commerce, which supported Strange. "They thought voting for Moore would not hurt the president's agenda, and they still wanted to upset the apple cart."

In the end, Alabama Republicans decided to go with the candidate who most resembled Trump's renegade spirit, even if it meant going against Trump's candidate. "Luther is the swamp," Carey Williams, 53, explained outside his polling place Tuesday in Mountain Brook, Ala. "I'm a huge Trump supporter, but I'm not going to follow him blindly."

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Such sentiments should send warning signs to other incumbent Republicans who are hoping the president could provide them cover in tough primaries next year. It's simply no longer clear if the White House can do much to protect Sen. Dean Heller (R-Nev.) or Sen. Roger Wicker (R-Miss.) as they prepare for the coming fights against anti-establishment primary candidates. Republican voters have made clear that they are less interested in policy positions than in finding candidates who, like Trump, promise to shake up the Washington establishment.

"President Trump can turn a zero to a hero with one tweet in a Republican primary," said Andy Surabian, a former deputy White House political strategist for Trump who traveled to Alabama to support Moore. "But it has to be a candidate that his supporters actually believe supports his agenda."

Surabian is now a senior adviser to the Great America Alliance, a group affiliated with Stephen K. Bannon, the former White House chief strategist. Like Bannon, Surabian has argued that the contest in Alabama was never a referendum on support for Trump. Instead, it was an test of the support Trump's partners in the congressional leadership in Washington could muster, and Strange wound up on the wrong side of that divide for too many voters.

In a memo on the race, Steven Law, the president of the Senate Leadership Fund, which spent about $9 million on the contest, told donors that primary voter support for Trump was "the dominant factor driving GOP voters." The fact that another primary challenger, Rep. Mo Brooks (R-Ala.), had once criticized Trump proved fatal to his campaign. "Support for President Trump directly correlates with likelihood to vote," Law wrote in the memo, which was first published by the New York Times.

During the final days of the campaign, Moore never said a bad word about Trump, beyond suggesting that the president had been duped by his advisers. The reason was plain to see in the polls, which showed that more than 80 percent of likely primary voters approved of the president.

Trump continues to get similar numbers among Republicans nationwide. In the most recent Washington Post-ABC News poll, which was conducted Sept. 18 through Sept. 21, 80 percent of Republicans said they approved of Trump's job performance, and 74 percent said Trump had brought needed change to Washington. Republicans even approved, by a margin of 77 percent to 14 percent, of Trump's recent decision to cut a deal with Democrats to raise the debt limit and fund hurricane disaster relief.

Though Trump stuck with his commitment to support Strange, he also in more subtle ways provided a safe space for his supporters to support Moore. He declined to distance himself from his former aide Bannon, who barnstormed the state saying a vote for Moore was a vote for Trump.

And at a final rally for Strange in Huntsville, Ala., Trump made clear that he would support Moore in the general election if he won. "I might have made a mistake," he admitted, in a surprising aside.

In the end, the vote count was not even close. Moore beat Strange by a 10-point margin, and as he promised, Trump promptly tweeted his congratulations. The president also deleted his older tweets endorsing Strange — a sign that it was one part of his record in office that he did not want used against him in the future.

Larry Bleiberg in Birmingham contributed to this report.

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