Frankly, We Did Win This Election: The Inside Story of How Trump Lost by Michael C. Bender:
It was a month after that Orlando rally, on July 25, when Trump picked up the phone and repeatedly pressured Ukrainian president Volodymyr Zelensky to help smear his political rivals. Trump wanted Zelensky to spread misinformation about Russian attempts to influence the 2016 election and investigate Joe Biden, the former Democratic vice president who had opened his campaign to unseat Trump just three months earlier. That phone call—paired with Trump’s attempt to force Zelensky’s hand by blocking $400 million in military aid for Kyiv—sparked the impeachment inquiry in late September.
The impeachment inquiry, in many ways, marked the actual start of the campaign. It provided purpose and mission to Trump World and offered a desperately needed reason for a fractured Republican Party to unite around the president. Trump always intended to treat his reelection in 2020 as a continuation of his campaign in 2016, even though an incumbent running as a change agent was a tricky feat to pull off. The impeachment allowed the president once again to cast himself as a victim and Washington outsider.
But the prospect of impeachment left him deeply rattled.