When President Barack Obama negotiated a nuclear deal with Iran more than a decade ago, his point man was Secretary of State John Kerry. Over 20 months of talks, Mr. Kerry met with his Iranian counterpart on at least 18 different days, often several times per day.

High-level nuclear diplomacy was a natural role for the top U.S. diplomat. Secretaries of state traditionally take the lead on the country’s biggest diplomatic tasks, from arms control treaties to Israeli-Palestinian agreements.

But as President Trump prepares to send a delegation to the latest round of U.S.-Iran talks in Pakistan this weekend, his secretary of state, Marco Rubio, will remain where he often does: at home.

Mr. Rubio did not attend the last U.S. meeting with Iran earlier this month. Nor did he join several meetings held over the past year in Geneva and Doha. Mr. Rubio has also been absent from U.S. delegations abroad working to settle the war in Ukraine and Israel’s war in Gaza. Despite a long period of crisis and war in the region, he has not visited the Middle East since a brief stop in Israel last October.

In recent months, Mr. Rubio — consumed with his second role, as Mr. Trump’s national security adviser — has not traveled much at all.

During the Biden administration, Secretary of State Antony J. Blinken made 11 foreign trips from January 2024 to late April 2024, stopping in roughly three dozen cities, according to the State Department. So far this year, Mr. Rubio has visited six foreign cities, including a stop in Milan for the 2026 Winter Olympics.

Mr. Trump has outsourced much of his diplomacy to others, including his friend Steve Witkoff, a wealthy associate from the world of Manhattan real estate, and his son-in-law, Jared Kushner. Mr. Witkoff and Mr. Kushner have spearheaded diplomacy with Israel, Ukraine and Russia, as well as Iran, whose delegation they will meet for the second time this month in Islamabad, Pakistan’s capital.

Mr. Rubio’s distance from the trenches of diplomacy reflects his dual role on Mr. Trump’s national security team. For the past year, he has served as the White House national security adviser even while leading the State Department — the first person to do so since Henry A. Kissinger in the mid-1970s.

The secretary of state runs the State Department, overseeing U.S. diplomats and embassies worldwide, as well as Washington-based policymakers. Working from the White House, the national security adviser coordinates departments and agencies, including the State Department, to develop policy advice for the president.

The twin roles reflect Mr. Rubio’s influence with Mr. Trump, and offer him a way to maintain it. For Mr. Rubio, less time abroad means more time at the side of an impulsive president prone to making critical national security decisions at any moment.

As Mr. Witkoff, Mr. Kushner and Vice President JD Vance met with Iranian officials in Pakistan earlier this month, Mr. Rubio was at Mr. Trump’s side at an Ultimate Fighting Championship event, noted Emma Ashford, an analyst of U.S. diplomacy at the nonpartisan Stimson Center in Washington. “Rubio clearly prefers to stay close to Trump,” Ms. Ashford said.

Mr. Rubio accepted the national security adviser job on an acting basis last May after Mr. Trump reassigned the job’s previous occupant, Michael Waltz. But officials say that Mr. Rubio is expected to keep it indefinitely.

That arrangement is not inherently bad, Ms. Ashford added. And she noted that previous presidents had entrusted major diplomatic tasks to people other than the secretary of state. President Joseph R. Biden Jr. delegated his C.I.A. director, William J. Burns, to handle diplomacy with Russia and cease-fire negotiations between Israel and Hamas, for instance.

But she echoed the complaints by many current and former diplomats that Mr. Rubio seems less like someone performing both jobs than a national security adviser who sometimes shows up at the State Department. “I do think it’s to the detriment of the whole department of State and to America’s ability to conduct diplomacy in general that we effectively have the secretary of state position sitting vacant,” she said.

Tommy Pigott, a State Department spokesman, contested such claims. “Anyone trying to paint Secretary Rubio’s close coordination with the White House and other agencies as a negative could not be more wrong,” he said. “We now have an N.S.C. and State Department that are totally in sync, a goal that has eluded past administrations for decades.”

Mr. Rubio divides his time between the State Department and the White House, often spending time at both in the same day. In an interview with Politico last June, Mr. Rubio said he visited the State Department “almost every day.”

While there, he often meets with visiting dignitaries before returning to the White House. Last week, Mr. Rubio presided over a meeting at the State Department between Lebanese and Israeli officials that set the stage for a cease-fire in Lebanon.

His twin jobs “really do overlap in many cases,” he said. “In many cases you end up being in the same meetings or in the same places; there’s just one less person in there, if you think about it,” Mr. Rubio added. “A lot of people would come to Washington, for example, for meetings, and they’d want to meet with the national security adviser and then meet with me as secretary of state. Now they can do both in one meeting.”

Asked about his travel schedule during a news conference last December, Mr. Rubio said he had less reason to travel abroad because “we have a lot of leaders constantly coming here” to visit Mr. Trump at the White House. Mr. Rubio also joins Mr. Trump’s foreign trips in his capacity as national security adviser.

Many national security veterans call the arrangement unwise, saying that both jobs are extremely demanding and incompatible with one another.

It was not easy even for Mr. Kissinger, who had firmly established himself over more than four years as national security adviser before convincing President Richard M. Nixon to let him take on an additional role as secretary of state in 1973. (In a reversal of Mr. Rubio’s approach, Mr. Kissinger was in constant motion, including a round of Middle East shuttle diplomacy that kept him on the road for 33 straight days.)

“In general, it’s a mistake to combine those roles,” said Matthew Waxman, who held senior roles at the National Security Council, State Department and the Pentagon during the George W. Bush administration.

“That said, it’s not necessarily a bad thing that a dual-hatted Rubio is so offscreen right now,” Mr. Waxman added. “Especially while so much attention is focused on high-wire diplomacy with Iran, someone needs to manage foreign policy around the rest of the world.”