“Maybe Donald negotiated a deal with his buddy Hillary Clinton,” Mr Bush tweeted this week, after Mr Trump cited a poll showing his supporters would stick with him if he left the Republican Party. “Continuing this path will put her in the White House.”
“If Donald Trump were a Democratic mole placed in the Republican Party to disrupt things, how would his behaviour be any different?” asked conservative political commentator George Will in July. “I don’t think it would be.”
Just over a week later Republican Congressman Carlos Curbelo of Florida called Mr Trump “a phantom candidate recruited by the left to create this entire political circus.”
Also suspicious – for those predisposed to suspicion, at least – is a “mystery” phone call between Mr Trump and Bill Clinton in May, less than a month before the real-estate tycoon tossed his hat into the presidential ring.
“Clinton encouraged Trump’s efforts to play a larger role in the Republican Party and offered his own views of the political landscape,” the Washington Post reported at the time.
Conservative commentator Brian Cates is less circumspect.
“Trump didn’t jump into this race because of his deep abiding love for America, or his being a Republican or caring about conservatism,” he writes. “Trump jumped into this race because BILL CLINTON urged him to.”
Noah Rothman of Commentary magazine spies a pattern in Mr Trump’s diatribes, whose timing, he argues, “tends to often coincide with scandalous revelations that reflect poorly on Democratic politicians”.