Senate Republican leaders, however, remained critical of Moore, warning that the former judge is likely to face an immediate ethics probe if he is elected next week.
The divergent attitudes toward Moore, who has been accused of making unwanted sexual advances toward teenage girls when he was in his 30s, underscored how polarizing a figure he would be among his party's national leaders if he wins the Dec. 12 special election.
Even if Moore is largely ostracized by his Senate colleagues, the support of the president could make him an influential figure in Washington — a point he appeared determined to emphasize on Monday.
"I look forward to fighting alongside the President to #MAGA!" Moore wrote on Twitter, using the acronym for Trump's signature campaign theme, "Make America Great Again."
Trump and Senate Republicans have already started pondering Moore's place in the party if he gets past Democrat Doug Jones in a contest that recent polling shows is neck and neck. The president wrote on Twitter on Monday that the united Democratic opposition to the GOP's sweeping tax plan showed "why we need Republican Roy Moore to win in Alabama."
"We need his vote on stopping crime, illegal immigration, Border Wall, Military, Pro Life, V.A., Judges 2nd Amendment and more. No to Jones, a Pelosi/Schumer Puppet!" Trump wrote.
Trump's message went further than his previous remarks about the race, in which he has bashed Jones but stopped just short of advocating for Moore.
The president placed a call to Moore on Monday morning, and the Senate nominee proudly trumpeted it in his own tweet.
"'Go get 'em, Roy!' - President Trump," Moore tweeted.
Trump's message came on the same day that the RNC reversed itself and returned to Alabama to support Moore, less than three weeks after pulling out of a joint fundraising agreement with his campaign. An RNC official with knowledge of the plans, speaking on the condition of anonymity, confirmed the re-engagement. The committee did not immediately issue any public statements announcing the move.
On Capitol Hill, the reception remained frostier — although a shift in tone appeared underway among senators, too. Sen. John Cornyn (Tex.), the second-ranking Republican senator, said Monday that he favors a congressional ethics probe into the allegations facing Moore if he is elected.
But Cornyn also hinted that if Moore wins, he would be a factor in the Senate and cannot be ignored.
"None of us get to vote on who's the senator from Alabama. Just Alabama voters do. So I think we have to respect their decision — whatever it is," he said.
That statement contrasted with a drumbeat among GOP senators for Moore to end his campaign just after The Washington Post reported the first allegations.
Led by Senate Majority Leader Mitch McConnell (R-Ky.), Republicans also explored ways to try to elect another GOP candidate, either through a write-in campaign or a postponed election. But neither option proved feasible, as local Republicans rallied around Moore and the White House refused to intervene.
Since then, McConnell and his top deputies have taken a more hands-off approach to the race. Like Cornyn did on Monday, McConnell said in weekend television interviews that the outcome was in Alabama's hands.
At the other end of Pennsylvania Avenue, Trump has been ramping up his involvement in the race. After staying relatively quiet in the immediate aftermath of the allegations, the president has steadily grown warmer toward Moore.
Before endorsing Moore on Monday morning, Trump had spoken at length about the Alabama race with his former chief strategist, Stephen K. Bannon, according to a person familiar with the conversation granted anonymity to describe private talks. Bannon was scheduled to appear at a Moore rally Tuesday afternoon in Fairhope, Ala.
Trump will travel to the same media market on Friday, just over the border in Pensacola, Fla., where he could encourage Alabamians to turn out for Moore.
One senior White House official said Trump jumped in for a few reasons: because aides convinced him that his support could push Moore to victory, because he would probably take part of the blame if Moore lost and because he didn't like the idea of backing Moore less than full-throatedly.
The race has exposed some tensions between RNC Chairwoman Ronna McDaniel and the White House, White House officials and advisers said. After Trump returned from Asia, he began expressing frustrations that the RNC backed away — even though he was apprised of the decision at the time.
McDaniel, according to one person close to her, felt boxed in and feels like the episode has been a bit of an embarrassment. She also wanted to quickly show the president that the RNC was loyal.
White House Chief of Staff John Kelly has been frustrated with the Alabama race and did not want the president to dive back in with a full endorsement, these officials said. His argument was one of practicality — that the White House didn't need to bother itself with the race and let that "become the focus," one of the people said. They spoke on the condition of anonymity to describe private discussions.
But Trump was shown polls in recent days that were positive for Moore and he became convinced that he would take some of the blame if Moore lost. Trump made the call that the White House should put the RNC behind Moore.
For those around Bannon, Trump's decision to embrace Moore less than a month after the RNC and the National Republican Senatorial Committee pulled out of the campaign is a victory in their fight against McConnell. Bannon is expected to once again mention McConnell in his remarks Tuesday night.
"This is a total embarrassment for Mitch McConnell, who put all of his political capital on the line to deliver this Senate seat to a liberal Democrat completely opposed to the Trump agenda," said Andy Surabian, a confidant of Bannon who serves as senior adviser to the Great America Alliance, a pro-Trump advocacy group.
McConnell's allies continue to view the aggressive defense of Moore mounted by Bannon as a warning sign for the rest of the party.
"You have this unbelievable contrast between on the one hand cash cows like Matt Lauer being shown the door, and on the other hand you have Steve Bannon sending down people to dig up dirt on women who come forward with serious and credible allegations," said Steven Law, a McConnell ally who runs the Senate Leadership Fund, which campaigned against Moore in the primary.
If Moore wins and is seated, he could face immediate scrutiny from the Senate Select Committee on Ethics, a normally secretive panel of three Republicans and three Democrats.
The panel, which has said it is looking into allegations of sexual misconduct against Sen. Al Franken (D-Minn.), could look into the accusations confronting Moore and determine their validity and whether they merit punishment such as a censure, or, in the most severe case, a vote in the full Senate over whether Moore should be expelled.
Rob Walker, a former chief counsel and staff director of the Senate and House Ethics panels, predicted the process could take three months or more.
Expelling a senator is very rare and would require the approval of two-thirds of the Senate. An actual vote hasn't happened since 1862.
A recent Washington Post-Schar School poll showed that 50 percent of likely Alabama voters support Jones and 47 percent back Moore, which is within the margin of error. On Monday, both candidates sought to shore up their support.
After a campaign event in his home town of Fairfield, Ala., Jones, who is trying to win over Republicans considering crossing party lines over the Moore allegations, parried questions about the Trump endorsement by saying he'd ignore "people calling me names" and focus on his own race.
"We're talking about issues. We're meeting people. We're talking to the media," Jones said. "Roy Moore is nowhere to be seen."
Asked if he would have supported the tax bill that passed the Senate last week, Jones said it would "blow up the deficit" and that there hadn't been enough bipartisan work to fix it.
Jones also attacked Bannon.
"President Trump ran Steve Bannon out of the White House because of his politics of division," said Jones. "It has no place in this state."
Should he win, Moore is expected to complicate McConnell's job of trying to keep Senate Republicans in line. He has openly criticized the GOP leader throughout his campaign.
Moore has also called the long-standing practice of requiring 60 votes for most legislation to pass the Senate "unconstitutional," and he has vowed to fight it if elected.
He also calls abortion a violation of the Constitution and has promised not to vote for a budget that would send any federal dollars to Planned Parenthood.
"I think I could help with the Judiciary Committee, because I do understand the Constitution," Moore said in an interview with One America News Network over the weekend that was broadcast Monday. "I do understand what judges do when they put themselves above the Constitution."
Even among a small group of conservative senators known for bucking McConnell, including Sens. Mike Lee (R-Utah) and Ted Cruz (R-Tex.), Moore could have trouble fitting in.
"Sen. Lee has unendorsed Judge Moore and called for him to step out of the race. Nothing has changed. Anything new on the issue would be premature at this point," Lee spokesman Conn Carroll said Monday.
Mitt Romney, the 2012 GOP presidential nominee and a potential Senate contender in Utah next year, wrote on Twitter that Moore becoming a senator "would be a stain on the GOP and on the nation."
Moore retorted that Romney had either "lost his courage or he doesn't care about truth anymore."
Sen. Richard C. Shelby (R-Ala.), who said he has already voted by write-in absentee ballot for an unidentified "distinguished Republican" who is not Moore, said in an interview Monday that he fears Moore's political style would not serve his state or the nation well.
"I think that he's thrived on controversy, and I don't believe anybody can thrive on controversy forever and not get burned up by it," Shelby said.
He added, "I don't believe that Roy Moore, if he was elected to the Senate, and he might be, would work in the Senate — I don't mean to go along to get along, I'm talking about to build the nation, build the state, at least as a lot of people believe we ought to do."
Weigel reported from Montgomery, Ala. Josh Dawsey in Salt Lake City and Philip Rucker in Washington contributed to this report.