Tributes poured in from across the world on Sunday for Senator Lindsey Graham, the longtime U.S. lawmaker and foreign policy hawk who died unexpectedly at age 71.

“Senator Lindsey Graham, one of the greatest people and Senators I have ever known, is dead!” President Trump wrote in a post on Truth Social. “He was always working, and was a true American Patriot. Lindsey will be greatly missed!!!”

Mr. Graham, a South Carolina Republican, died on Saturday after a “brief and sudden illness,” his office said. Once one of Mr. Trump’s harshest critics, Mr. Graham had refashioned himself in recent years as one of the president’s most devoted backers.

Across Mr. Graham’s many political lives, his unyielding endorsement of American military intervention was perhaps his most consistent cause. Most recently, Mr. Graham carried the torch for the U.S. war against Iran and encouraged aggressive support for Ukraine in its defense against the Russian invasion.

President Volodymyr Zelensky of Ukraine said that he had met with Mr. Graham just last week in Kyiv and was “deeply saddened” by his death.

“Lindsey was a true defender of freedom and the values that make our world safer,” Mr. Zelensky said in a social media post, adding that Mr. Graham had visited Ukraine 10 times since Russia’s full-scale invasion in 2022.

“We remained in constant dialogue, and I will miss our conversations. We met twice in just the past week.”

Mr. Graham’s firm defense of NATO was one of his rare breaks with Mr. Trump, who has long been critical of the alliance. Mark Rutte, the alliance’s secretary general, said that he was “so sad to learn of the sudden passing of my friend,” Mr. Graham.

“He was a powerful advocate for America who believed strongly in the NATO Alliance and was actively working to bring an end to Russia’s war against Ukraine,” Mr. Rutte said on social media.

Chancellor Friedrich Merz of Germany called Mr. Graham “a true friend and partner of Germany” and added: “We stood side by side for more than four decades.”

Mr. Graham was also a staunch defender of Israel, making numerous visits to that country. Israel’s prime minister, Benjamin Netanyahu, called him “a beloved friend.”

“Lindsey understood that the security of Israel and America are inseparable,” Mr. Netanyahu said in a post on X. “He devoted his life to defending America, strengthening our alliance and standing up for the free world.”

Mr. Graham shared a decades-long friendship with the late Senator John McCain of Arizona, a fellow Republican who also advocated for an interventionist U.S. foreign policy.

“From the time he met my Dad they were fast friends and political comrades,” Mr. McCain’s daughter Megan McCain wrote on X, recalling his “acerbic quick wit” and fondness for storytelling.

“He was literally never, ever boring,” she said.

Mr. Graham’s reinvention, in what would be the twilight of his career, as one of Mr. Trump’s fiercest defenders was jarring to those who regarded him as one of the Senate’s more likable firebrands, one who seemingly relished the bipartisan sparring of Washington. Even among those with whom he often vehemently disagreed, he was remembered fondly.

“Sen. Graham and I disagreed on almost everything, and he’d be the first to tell you that with a smile,” said Donna Brazile, the former head of the Democratic National Committee. “But he served this country in uniform and in the Senate. He believed deeply in America’s role in the world.”

Mr. Graham, who served as a lawyer in the U.S. Air Force before entering politics in the early 1980s, was scheduled to appear on NBC’s “Meet The Press” on Sunday morning. It would have been his 64th appearance on the program, its moderator, Kristen Welker, said.