"This won’t be the first time I’ve voted for a Democrat — though not for president [before]. Last time I voted for a third-party candidate. ... But I will not vote for Donald Trump," Mr Flake said in an interview with The Washington Post.
Mr Flake insisted that he is "not trying to burn the place down or anything else," but that he's trying to get back what he feels is the lost soul of the Republican party.
The best thing for the future of the Republican party would be "a sound defeat" for Mr Trump in November, Mr Flake said. "No doubt. Long term for the Republican Party, you bet. And for conservatism as well."
By the end of his first and only term as senator from 2013 to 2019, Mr Flake had the worst relationship with Mr Trump of any Senate Republican, as he openly and consistently denounced many of the president's controversial remarks and opposed a handful of his political and judicial appointments.
The men's relationship was never strong. Mr Flake called on Mr Trump to withdraw from the 2016 presidential race after the emergence of the Access Hollywood tape where Mr Trump openly brags about sexually assaulting women. Mr Trump was so enraged with Mr Flake that his White House was recruiting GOP primary challengers against the senator in the summer of 2017.
While many Republican senators who were critical of Mr Trump early in the 2016 campaign, such as South Carolina's Lindsey Graham, eventually fell in line with the president and have become some of his closest political allies, Mr Flake remained defiantly on the periphery. When he announced his retirement from the Senate in the fall of 2018 amid politically untenable approval ratings, he delivered a scathing speech against the state of politics in Washington that spared no one, not least of all his Republican colleagues nor Mr Trump.
The GOP's aggressive public embrace of the president's hard-nosed political style isn't a Washington-only phenomenon, Mr Flake lamented in his interview with the Post. Trumpism has ensnared Mr Flake's home state politicians, too, including many people to whom the former senator has long personal and political ties.
Arizona Republicans have been complicit in a "total capitulation of the party to Trumpism," Mr Flake vented in the Post interview.
In February, more than 14,000 people flocked to the Veterans Memorial Coliseum in Phoenix for one of the president's notoriously lively and raucous campaign rallies. In years past, many Arizona lawmakers were reticent about appearing onstage with the Mr Trump. That's no longer the case.
"The other night it was painful to watch the rally in Arizona: the president onstage with virtually all of my Republican colleagues from Arizona — the governor on down, some of whom had been reluctant previously to be on a campaign stage with the president. But who have just completely and utterly thrown in," Mr Flake said.
It is not all gloom and doom for his party, though, Mr Flake suggested. In private conversations, many Republicans acknowledge they've submitted to a "trade-off" in which they publicly tolerate Mr Trump's bombast and anti-institutionalism in exchange for his signature on conservative policies, his nomination of conservative judges and his enactment of tax and regulatory reform. That trade-off is not intended to last forever.
Mr Flake expressed confidence that the GOP can veer back towards a vision that is more inclusive and not fuelled, in his own words, by "anger and resentment."
"I don’t know anyone who thinks that this is the future of the party. This is a demographic cul-de-sac we’re in, if nothing else. Anger and resentment only go so far; you have to have a governing philosophy. I don’t know of any of my colleagues who really believe this is it."