Senator Lindsey Graham, Republican of South Carolina, likely died on Saturday from a tear in his aorta, officials said, hours after returning to the United States from Ukraine and months before he was to face re-election.

Mr. Graham’s office said on Sunday that a preliminary report from Office of the Chief Medical Examiner in Washington indicated that the senator had died from an aortic dissection, though that determination was subject to additional testing.

Mr. Graham, who was chairman of the Budget Committee and formerly led the Judiciary Committee, became a household name in Republican politics and was among President Trump’s closest and most loyal allies in the Senate. He died two days after his 71st birthday, and as he was seeking a fifth term.

A former Air Force lawyer who had served in the Air Force Reserve, Mr. Graham also developed a reputation as a foreign policy hawk who vehemently defended Israel and Ukraine. Hours before he died, the senator had returned to Washington after a trip to Ukraine, where he said he had reached agreement with colleagues in both parties and the White House on imposing sanctions targeting buyers of Russian oil.

Here’s what to know about the end of Mr. Graham’s life:

Emergency workers were called to his home.

Mr. Graham’s office did not provide any further details about what happened in the moments leading up to his death. But the medical examiner’s preliminary finding said he suffered a tear in the main artery that carries blood from the heart, “stemming from arteriosclerotic cardiovascular disease,” or a gradual weakening and hardening of the arteries.

According to recordings of dispatcher calls obtained by The New York Times, emergency workers responded on Saturday night to a call about a person experiencing chest pains at the senator’s Capitol Hill address. The recordings did not name Mr. Graham, who had returned from Ukraine hours earlier, but responders could be heard saying they were administering CPR to a person suffering from cardiac arrest.

According to the recordings, the emergency call for help came from a woman in Baltimore who was not identified. She informed responders that the house’s door was open, but emergency crews could be heard saying they needed the police to assist a forced entry.

Mr. Trump said in an interview on Sunday that he had spoken with Mr. Graham not long before the episode, and gotten no inkling that there was anything amiss.

“Other than being tired, he was fine,” Mr. Trump said. He said early reports of a possible heart attack made sense. “That would just be a quick end. And maybe that’s not the worst way to go.”

Officials with the D.C. Metropolitan Police and the Fire and Emergency Medical Services Department did not respond to requests for comment.

He had just returned from Ukraine.

In his last public act, on Friday, Mr. Graham announced during a visit to Ukraine that he had reached agreement among a bipartisan group of senators and the White House on his long-stalled effort, resisted by Mr. Trump, to impose sanctions targeting buyers of Russian oil.

It was a fitting end to the senator’s career on Capitol Hill, where he had long been a proponent of U.S. military intervention against adversaries abroad, particularly Russia. Last year, he had pressed for new, harsher sanctions on countries that did business with Moscow, but he pulled back amid Mr. Trump’s opposition.

“I have never been more optimistic than I am today that we have the formula to end this war,” Mr. Graham said on Friday in Kyiv, the Ukrainian capital. “Help Ukraine be more lethal. Let those supporting Russia to know there’s going to be a price to be paid if you keep doing it. And to try to find an off-ramp — not to humiliate Putin, but to end this war so that Ukraine will thrive and survive.”

President Volodymyr Zelensky of Ukraine said on Friday that he was grateful to Mr. Graham, posting a photo of the two shaking hands in Ukraine.

His trip to Ukraine came just days after Mr. Graham had traveled to Ankara, Turkey, for the NATO summit.

Officials will move quickly to replace him.

Mr. Graham’s death came a month after he won the Republican primary in South Carolina’s Senate race, and about four months before the general election he was heavily favored to win.

Between now and then, state law allows Gov. Henry McMaster, a Republican, to appoint a replacement to serve out the rest of Mr. Graham’s term, through Jan. 3, 2027. On Sunday, Mr. Trump said he already had someone in mind for the post.

A spokeswoman for the governor declined to answer questions about the process for filling the vacancy.

But the law sets out a quick timeline, indicating that a special primary election to replace Mr. Graham on the ballot would be held on Aug. 11, with candidates required to file for that contest between July 21 and July 28.

Plans are underway to honor him.

Mr. Trump announced on social media on Sunday that he would order flags to be flown at half-staff until Saturday evening in honor of Mr. Graham.

Arrangements on Capitol Hill have yet to be announced, but Senator Tim Scott, his fellow South Carolina Republican, suggested in an interview on CBS’s “Face the Nation” that G.O.P. leaders were planning to find ways of honoring Mr. Graham in the coming week.

It was not known whether he would lie in state in the Capitol, as his close friend and former Senator John McCain of Arizona did in 2018 after his death from a malignant brain tumor.

Bayliss Wagner and Robert Jimison contributed reporting.