For Republican Sen. Jeff Flake of Arizona, the other shoe just dropped. For the past week Flake, one of the Senate’s most vulnerable members up for re-election in 2018, has been taking to the airwaves to flog President Trump and promote his book, “The Conscience of a Conservative: A Rejection of Destructive Politics and a Return Principle.”

But Flake had to know that scoring political points against the president would have a price, and he’s getting the first major taste of it in the form of a $300,000 donation to his primary opponent from Trump ally and billionaire Robert Mercer.

First reported by Politico, the contribution to KelliPAC, a super PAC supporting conservative primary challenger Kelli Ward in the 2018 Senate election, is just the latest instance of Trump and his allies attacking members of his own party in Congress.

This fight comes after Flake, who has always had a rocky relationship with Trump, published his book last week criticizing Trump’s tweets as “all noise and no signal” and chiding the president for his “volatile unpredictability.”

He writes: “Far from conservative, the president’s comportment was rather a study in the importance of conflict in reality television — that once you introduce conflict, you cannot de-escalate conflict. You must continually escalate.”

Well, Flake called that one.

Mercer’s contribution to unseat Flake — a vote Trump needs in the Senate with only a 52-vote majority — is nothing if not an escalation of a conflict that began during the campaign. After Trump clinched the Republican nomination last summer, Flake confronted Trump in a meeting with fellow Republican Senators about his mocking of fellow Arizonan John McCain for getting captured during the Vietnam War. After the meeting, Flake told reporters that “I just can’t support [Trump] given the things that he’s said.”

The relationship went downhill from there. Trump threatened to spend $10 million of his own money to defeat Flake in 2018 and his White House has met with potential primary opponents.

And Flake isn’t the only one Trump has been picking fights with. A Trump-affiliated outside group launched TV and radio ads against Republican Sen. Dean Heller of Nevada — also up for reelection in 2018 —after he opposed one of the Trumpcare bills.

Even Senate Majority Leader Mitch McConnell was in Trump’s crosshairs this week after he said the president came into office with “excessive expectations” of what could be accomplished by Congress. Trump, naturally, escalated in a series of tweets.

Such blatant confrontations carry tremendous risks for the president and could ultimately diminish his influence on Capitol Hill and imperil his legislative agenda.

Trump’s biggest asset in these high-stakes showdowns is that he remains very popular with the Republican base and much more popular than leaders like McConnell and House Speaker Paul Ryan. Politicians fear losing, and the base turning on them is a quick route to that fate.