As the war in Iran threatens to imperil President Donald Trump's legacy, the political stakes also are rising for two of his top lieutenants: Vice President JD Vance and Secretary of State Marco Rubio. 

The pair, widely viewed as potential successors to Trump, have been thrust into still-developing negotiations to end the war at a moment when the Republican Party is already weighing its post-Trump future.

Vance has taken a cautious approach, reflecting his skepticism toward prolonged US military involvement, while Rubio has aligned himself closely with Trump’s hawkish stance and emerged as one of the administration’s most vocal defenders of the campaign.

Trump has said both men were involved in efforts to force Iran to accept US demands to dismantle its nuclear and ballistic missile programs and allow oil traffic to pass freely through the Strait of Hormuz.

With the next presidential election due in 2028 and term limits barring Trump from running again, the president has been putting the succession question to allies and advisers in private, asking "JD or Marco?" two people familiar with his views said.

The outcome of the US military operation now in its fifth week could shape the two men's 2028 prospects, political analysts and Republican officials said. A swift end to the war that favors the US might bolster Rubio, who also serves as Trump's national security adviser and could be seen as a steady hand during a crisis. A prolonged conflict could give Vance space to argue he reflected the anti-war instincts of Trump’s base without openly breaking with the president.

Trump's own standing is also at stake. His approval rating fell in recent days to 36%, its lowest point since he returned to the White House, hit by a surge in fuel prices and widespread disapproval of the ‌Iran war, ⁠a four-day Reuters/Ipsos poll completed last week found.

Some Republicans say they are watching closely for which senior aide Trump appears to favor as the Iran conflict unfolds. Some see signs of Trump leaning toward Rubio, but note he could change his mind quickly.

"Everyone is watching the body language that Trump makes on Rubio and not seeing the same on Vance," a Republican with close ties to the White House said.

The White House rejected the idea that Trump is signaling a preference.

"No amount of crazed media speculation about Vice President Vance and Secretary Rubio will deter this administration's mission of fighting for the American people," spokesman Steven Cheung said.

Vance, 41, a former Marine who served in Iraq, has long argued against US entanglements in foreign wars. His public comments on Iran have been limited and calibrated, and Trump has noted the two have "philosophical differences" on the conflict.

Once a self-described "never-Trumper," Vance wrote an opinion piece in the Wall Street Journal in 2023 saying Trump's best foreign ​policy was not starting any wars during his first four ​years in ⁠office between 2017 and 2021.

The White House has downplayed any rift between the president and vice president. Standing alongside Trump in the Oval Office earlier this month, Vance said he supported Trump's handling of the war and agreed with him that Iran should not obtain a nuclear weapon.

Vance could take on a more direct role in negotiations if Trump's special envoy Steve Witkoff and son-in-law Jared Kushner make sufficient progress, a person with knowledge of the matter said.

"Vice President Vance is proud to be a part of a highly effective team that, under President Trump’s bold leadership, has had incredible success in making America safer, more secure, and more prosperous," a Vance spokeswoman said.

A senior White House official, who, like others in this story, was granted anonymity to speak freely about a sensitive topic, said Trump tolerates ideological differences as long as aides remain loyal, adding that Vance's skeptical views have helped inform Trump about where part of his voter base stands.

A person familiar with Vance's views told Reuters the vice president will wait until after the November midterm elections before deciding on whether to run in 2028.

Rubio, 54, has said he will not run for president if Vance does, and sources familiar with Rubio's views say he would be content as Vance's running mate.

But any perceived vulnerability for Vance could encourage Rubio and other Republicans eyeing bids.

"Trump has a long memory," said Republican strategist Ron Bonjean. "And he may call out Vance for his lack of allegiance. And if Trump remains popular with the MAGA base, that could hurt him by not getting the endorsement of the president."

Trump has floated the idea of Vance and Rubio running together, suggesting they would be hard to beat.

"Trump doesn’t want to anoint anyone," the senior White House official said.

A March Reuters/Ipsos poll found that 79% of Republicans held a favorable view of Vance, while 19% held a negative view. Some 71% had a positive view of Rubio, while 15% viewed him unfavorably.

In comparison, 79% of Republicans viewed Trump favorably and 20% unfavorably.

Rubio, whose 2016 presidential aspirations were snuffed out after a bitter confrontation with Trump, has long since set aside any frictions with the president.

State Department spokesman Tommy Pigott said Rubio "has a great relationship, both professionally and personally," with Trump's team.

Rubio and the White House were forced into damage control after he angered some of Trump's conservative backers when he suggested that Israel pushed the United States into the war. But in the weeks since, Trump has praised Rubio's efforts.

Asked whether Rubio was concerned that a protracted war might damage his political future, a senior State Department official said, "He has not spent a second thinking about this.”

Matt Schlapp, a conservative leader who runs the Conservative Political Action Conference, said the Iran campaign will have big political consequences.

"If it is seen as successful at getting the job done...I think people will be politically rewarded for doing the right thing," Schlapp said. "If it goes on and on and on... I think the politics are tough."

Republicans remain broadly supportive of the US military strikes against Iran, with 75% approving compared to just 6% of Democrats and 24% of independents, Reuters/Ipsos polling showed.

At a televised Cabinet meeting on Thursday, the contrast between Rubio and Vance was on display.

Rubio gave a full-throated defense of Trump's attack on Iran. "He's not going to leave a danger like this in place," the Secretary of State said.

Vance was more measured, focusing on options for depriving Iran of a nuclear weapon. He closed by wishing Christians and US troops in the Gulf a blessed Holy Week and Easter.

"We continue to stand behind you," he said to servicemembers, "and continue to support you every step of the way."