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.

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: