Challenging Trump's refusal to pledge support for the eventual nominee should have been a moment that earned Paul some credibility among Republicans frustrated with Trump's rise.

But it ended up falling flat.

"It just missed the mark," the strategist said. "He didn't give off a good vibe doing it."

Last fall, Time magazine declared Paul the "most interesting man in politics" and stamped him on its cover. Paul launched his campaign earlier this year pledging that he was a "different kind of Republican."

Four months later, though it's still early in a crowded, fluid race, it's clear that many Republicans want different — but not him.

His campaign is struggling to keep up with his rivals in fundraising. Two of his political allies running an outside super PAC supporting his candidacy were recently indicted on campaign-finance fraud charges. Significant plunges in polling are starting to correspond.