At a raucous rally in Huntsville, the President warned that that former judge Moore could lose the state's Senate special election to a Democrat -- but if Strange, who already holds the Senate seat, wins Tuesday's primary, "it's over."
With the endorsement, Trump took what he admitted was a risk -- pumping his own political capital into an effort to carry Strange to victory, even though a Moore win could put a dent in Trump's influence.
Trump said he "might've made a mistake, I'll be honest," by wading into the race.
But, he said, "Luther's going to win easily and Roy is going to have a hard time winning" a December general election against Democrat Doug Jones.
Trump's comments about electability came as he acknowledged that Senate Republicans can't spare a single vote. He pointed to Sen. John McCain's announcement Friday that he opposes the latest GOP health care bill -- effectively ending the party's chances for now at repealing Obamacare.
His decision to visit Alabama and campaign for Strange, Trump said, was all about loyalty.
The President bluntly told the crowd that when he was handed a list of 10 senators he needed to lobby to support his health care effort, some insisted on dinners and meetings with their families before Trump could win them over.
But when he called Strange, the new senator appointed in February to replace Attorney General Jeff Sessions, said, in Trump's retelling: "Sir, don't even waste your time talking anymore. You have a lot of business to do. You have my vote."
Trump said he told first lady Melania Trump about the call. "I went home and told my wife, that's the coolest thing that's happened to me in six months."
Trump also defended Strange against Moore's accusations that he is doing Senate Majority Leader Mitch McConnell's bidding, saying that Strange has only known McConnell for months. The charges have stemmed from a pro-McConnell super PAC spending $9 million to boost Strange.
Trump and McConnell have butted heads as Republicans' frustration has mounted over the party's failure to repeal Obamacare and enact other Trump agenda items, with Trump publicly blasting the Senate leader on Twitter repeatedly in recent weeks.
At the rally, Trump said he knew Strange had trailed in the polls. But he said: "I think you're going to come back and you're going to kick everyone's ass and you're going to be great. You're going to be great. Because he got saddled with things he should not have gotten saddled with."
Still, Trump's hedges -- at one point he said he'd support Moore if he wins the primary -- reflected a White House still cautious about Strange's chances against a figure with decades-long relationships in Alabama politics. Moore, who campaigns on placing Christianity at the center of public life and opposing LGBT rights, was twice ousted as state Supreme Court chief justice -- first for refusing to remove a Ten Commandments monument then for refusing to recognize the US Supreme Court's decision legalizing same-sex marriage.
Health care struggles
Trump blasted McCain at the Friday night rally, calling the Arizona Republican's choice to oppose the latest GOP effort to repeal Obamacare "honestly terrible" at a rally in Alabama.
Recalling the list of 10 "no" votes he was told he needed to court, Trump said, "John McCain was not on the list, so that was an unexpected thing. Terrible. Honestly terrible."
Trump added that McCain's "last campaign was all about repeal and replace, repeal and replace."
"We're going to do it eventually," Trump vowed, referring to his pledge to repeal former President Barack Obama's Affordable Care Act.
Trump added that he still believes Kentucky Sen. Rand Paul, who like McCain has said he will vote against the bill, could change his mind.
"I haven't given up on him, because I think he may come around, OK? Wouldn't it be ironic if he took John McCain's place -- and they definitely do not like each other. Wouldn't that be ironic?" Trump said. "That would be very ironic, for those of you that know the inner workings of the Senate."
And he railed against the Senate's "crazy" 60-vote threshold to pass legislation, complaining that -- with Republicans controlling only 52 seats -- it's impossible to pass bills. He said the GOP can pass bills under special budget reconciliation rules that require just 51 votes, "but that's a trick. That's just a trick."
The rally had some of Trump's hallmarks. He claimed that audience members would be "turning over your rifle" if Democratic nominee Hillary Clinton had won the 2016 election. And when the audience chanted "lock her up," he responded that they should talk to Sessions about it.
Trump also said he'd like to see NFL owners respond to players kneeling
during the National Anthem by saying: "Get that son of a bitch off the field right now, he's fired. He's fired!"
"For a week, (that owner would) be the most popular person in this country," Trump said, "because that's a total disrespect of our heritage. That's a total disrespect for everything we stand for.
And he complained about NFL efforts to enforce penalties designed to prevent concussions. "They're ruining the game, right?" he said. "They're ruining the game."
'Russia did not help me'
Trump called the investigation into Russian meddling in the 2016 election "one of the great hoaxes," and said it is an "excuse" for Democrats after they lost the contest.
"Honestly, it's the thing they did best," Trump said of Democrats. "They did a rotten job of running, but they convinced people of this hoax. That was probably the thing that they did best, but it was one great hoax."
He added: "No, Russia did not help me, that I can tell you, OK?"
Trump also bragged about his victory, touting his focus on electoral votes rather than the popular vote, and said he hopes Clinton runs again in 2020.
"If Hillary runs again in four years, which I hope she does, we're going to teach her to spend more time in Michigan. We're going to teach her to go to Wisconsin," Trump said.
'Little Rocket Man'
The trip south comes after Trump spent a stiff week in New York City
meeting world leaders at the United Nations, which he called "very productive" at Friday's rally.
Trump took aim again at North Korean dictator Kim Jong Un's nuclear aspirations, calling him "Little Rocket Man" and promising that "we're going to handle it, folks" at a rally in Alabama.
"We can't have mad men out there shooting rockets all over the place. And by the way, Rocket Man should have been handled a long time ago," Trump said, adding that former President Barack Obama and Hillary Clinton should have dealt with North Korea during Obama's tenure. "He should have been handled eight years ago, and four years ago, and honestly -- and 15 years ago, and 20 years ago, and 25 years ago. This shouldn't be handled now, but I'm going to handle it because we have to handle it."
Trump touted his administration's executive accomplishments, including withdrawing from the Trans-Pacific Partnership and advancing the Keystone XL pipeline. He pointed to efforts to reform the Veterans Affairs department and increase defense spending.
Of the Obama administration's Clean Power Plan, Trump said: "Did you see what I did to that? Boom, gone."
He pitched his next major legislative agenda item, tax reform, saying that it will be "the biggest tax cut" and that he'd rescind his endorsement if Strange didn't support it.
Trump, Bannon face off
The Alabama Senate primary marks the first time Trump is directly confronting his former chief strategist, Steve Bannon, who is aggressively supporting Moore while framing the primary as a chance to reject McConnell. Bannon has been rallying anti-establishment conservative donors and influencers and using the conservative news site Breitbart.com to attack Strange.
But Strange used Bannon's support to poke at Moore at a debate on Thursday night.
"Many of the people who are supporting you look like the unemployment line at the White House," he said. "They were fired."