Graham's office announced his death in a statement, saying the South Carolina senator died unexpectedly just a day after returning from Kyiv, where he had met Ukrainian President Volodymyr Zelensky. There had been no known health concerns before his trip.

President Trump led tributes to the longtime senator, describing him on social media as a "true American Patriot."

Graham, first elected to the Senate in 2002, became one of Trump's strongest political supporters despite years of public disagreements between the two.

His sudden death comes as he was working on a new version of a Russia sanctions bill, which he said would give "tools to President Trump to end this war."

Graham remained one of Washington's strongest advocates for Ukraine.

Zelensky said he was "deeply saddened" by the senator's death, praising Graham's commitment to Ukraine since Russia launched its full-scale invasion.

The Ukrainian president said Graham had visited the country 10 times during the war and was "with our people when it was most needed."

"America and the world have lost a determined leader," Zelensky added.

Graham's political relationship with Trump changed dramatically over the past decade. During the 2016 presidential campaign, he was one of Trump's Republican critics.

He told CNN in 2015, according to media reports that Trump was "a race-baiting, xenophobic, religious bigot", and later warned on social media: "If we nominate Trump, we will get destroyed... and we will deserve it."

Following the January 6, 2021, attack on the US Capitol, Graham publicly distanced himself from the president, telling fellow senators: "Trump and I, we've had a hell of a journey. I hate it to end this way.

"All I can say is a count me out. Enough is enough."

However, the senator later rebuilt his alliance with Trump. He voted against convicting him during the 2021 impeachment trial and backed his successful 2024 presidential campaign.

Explaining his support in a 2023 interview with the BBC, Graham said: "There is a dark side to Donald Trump... and he was a very good president. But I am sticking with him because I saw what he did."

He pointed to Trump's immigration policies, the killing of Iranian military commander Qasem Soleimani and the appointment of conservative judges as key achievements.

Throughout more than two decades in the Senate, Graham earned a reputation as a leading advocate of an interventionist US foreign policy.

According to his official website, he "consistently pushed for outcomes in the War on Terror that protect our long-term national security interests."

He supported the US-led military action in Iraq after the September 11, 2001, terrorist attacks and strongly opposed the withdrawal of American troops from Afghanistan in 2021, calling it a "sad and dangerous event for US national security."

"Jihadists all over the world are celebrating," he added. "America will be seen as weak."

Graham was also among Israel's strongest supporters in the US Senate.

Israeli President Benjamin Netanyahu paid tribute on Sunday, saying, "Lindsey understood that the security of Israel and America are inseparable."

Israel had lost "one of its greatest friends," Netanyahu added.

Graham's death marks the loss of one of the Republican Party's most influential lawmakers, whose career shaped debates on national security, military intervention and America's role in global conflicts for more than two decades.