High regard for Pence among congressional lawmakers could be an especially tender pressure point for the president. One senior Republican Senate staffer, who requested anonymity to describe the situation candidly, told me, “If it was just a matter of magically snapping their fingers … pretty much every Republican senator would switch out Pence for Trump. That’s been true since day one.”

This week isn’t the first time we’ve seen signs of a potential rift. Last year, The New York Times reported that Trump was privately asking allies whether they thought Pence was loyal. More recently, rumors have circulated that the president might replace Pence on the 2020 ticket with former United Nations Ambassador Nikki Haley. And in a news conference last week, Trump voluntarily dragged his vice president into the Ukraine mess by suggesting that journalists look into the calls Pence had made to President Volodymyr Zelensky.

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But the roots of Trump’s distrust can be traced back to the final weeks of the 2016 election, when the Access Hollywood tape became public. In the uproar that followed, Pence did not jump to his running mate’s defense. Instead, he retreated from the campaign and issued a disapproving statement: “I do not condone his remarks and cannot defend them.” Anonymous quotes began popping up in the press describing Pence as “beside himself” over Trump’s remarks. And as I would later report in a profile of the vice president, Pence made it clear to the Republican National Committee that he was ready to take Trump’s place at the top of the ticket. (A spokesperson for Pence has denied this.) I wrote:

Republican donors and party leaders began buzzing about making Pence the nominee and drafting Condoleezza Rice as his running mate. Amid the chaos, Trump convened a meeting of his top advisers in his Manhattan penthouse. He went around the room and asked each person for his damage assessment. [Reince] Priebus bluntly told Trump he could either drop out immediately or lose in a historic landslide. According to someone who was present, Priebus added that Pence and Rice were “ready to step in.”

This episode has not been forgotten by certain Trump loyalists, who believe Pence has shown he will prioritize his own political interests over the president’s if necessary. As one former Trump adviser told me in 2017, “I don’t think he goes down with the ship.”

Of course, the ship is still a long way from sinking. Even if the House impeaches Trump, it remains unlikely that the Senate will vote to remove him from office. But many of the traits that make Pence valuable to Trump now could make him a threat if the dynamics change.

On Capitol Hill, where Trump’s fate may be decided, Pence is far more popular than the president. Doug Heye, a Republican strategist who worked for more than a decade in Congress, told me the lawmakers he’s talked to are exhausted by the president’s behavior. “Everywhere they go, there’s a mic in their face and a reporter saying, ‘Defend what Trump just did,’” Heye said. If Pence ascended to the Oval Office, “it would make the lives of every Republican member easier.”