How Trump's alliance with powerful Republican senator broke down into verbal warfare

— -- President Donald Trump’s war of words with a powerful Republican senator escalated over the weekend, with the president attacking Sen. Bob Corker of Tennessee in a series of tweets and culminating in a bombshell interview in which the GOP lawmaker said Trump was setting the U.S. on a “path to World War III.”

Just last year, Trump and Corker, chairman of the Senate Foreign Relations Committee, were allies.

Corker, who was a real estate developer as Trump is, rallied with Trump during the presidential campaign and endorsed him for the GOP nomination ahead of the Republican National Convention.

“The reason you love him so much is because he loves you,” Corker told a crowd at a North Carolina rally prior to the convention. “He loves you, and he wants the best for you.”

The senator added, "It says a lot about a person to meet their family, spend time with their kids, if you will, and to be around the people that have worked in the Trump organization for 25 and 30 years, to see the respect they have for the person they have worked with … to see how he treats people around him.

Trump returned the praise, calling Corker “a great friend of mine, somebody respected by everybody.”

Corker was at one point even considered as a potential running mate for Trump.

He didn't seem interested, however, telling The Washington Post, “There are people far more suited for being a candidate for vice president, and I think I’m far more suited for other types of things.”

Two days after Trump won the presidential election in November, Corker told ABC News that he was "excited about the opportunity that awaits President-elect Trump, Congress, and the American people."

By August of this year, however, Corker had some critical words after the president seemed to suggest a moral equivalence between white nationalists and counterprotesters in the wake of deadly violence in Charlottesville, Virginia.

The president denounced "hatred, bigotry and violence on many sides."

Corker said Trump was "helping inspire divisions" in the nation because that "generates support" from his political base.

Trump "has not demonstrated that he understands the character of this nation," the Republican senator said. "The president has not yet been able to demonstrate the stability nor some of the competence that he needs to demonstrate that he needs to be successful."

Corker called for "radical changes" from the White House.

The following month, though, the Tennessee lawmaker again heaped praise on Trump the day after his address to the United Nations General Assembly on Sept. 19.

“President Trump sent a very clear signal yesterday about what America stands for and who and what the world must stand against,” said Corker. “After years of ambiguous messaging, the president’s direct approach was a welcomed change, and I shared that with him when we spoke this morning.”

But last week, following a report that Secretary of State Rex Tillerson had allegedly referred to the president months before as a "moron," Corker praised Tillerson along with Defense Secretary James Mattis and White House chief of staff John Kelly.

"I think Sec. Tillerson, Sec. Mattis, and Chief of Staff Kelly are those people that help separate our country from chaos,” Corker said Wednesday.

Things got more heated from there.

Trump used Twitter on Sunday morning to slam the Tennessee senator, whose second term in the Senate ends next year and who has said he won't run for a third term.

The president claimed in his tweets that Corker decided not to run again after Trump said he wouldn't endorse him and that the senator had wanted the post of secretary of state.

Corker responded sharply, tweeting, "It's a shame the White House has become an adult day care center."

And, in a later statement to ABC News, the senator's chief of staff, Todd Womack, disputed the president's tweeted claims.

"The president called Senator Corker on Monday afternoon and asked him to reconsider his decision not to seek reelection and reaffirmed that he would have endorsed him, as he has said many times," Womack said.

In a subsequent interview Sunday with The New York Times, Corker said Trump was behaving as if he was on “a reality show,” had undermined diplomacy with his tweets and could put the U.S. “on the path to World War III.”

Corker will be in office until his term ends in early January 2019. Despite his decision against running for reelection, he could be a crucial vote in the Senate over the next 15 months on issues such as tax reform.

“I've provided the entertainment for all of you because I’m willing to say things that are on people's minds,” Corker said in an interview with ABC News in September, a day after he announced that he would not seek reelection.

“I’m frank and I’ve been that way because I’ve never thought about a future election. It’s always been about trying to make something happen,” he said.

ABC News' Alexander Mallin contributed to this report.