“My own personal view is I don’t think Rubio is ready,” said Alex Vogel, who was a top fundraiser for Mitt Romney in 2012, but has not yet committed to a candidate in this cycle. “I don’t think he’s ready, and Jeb does create a serious problem.”

Rubio is also struggling to gain support among blocs in the GOP where he previously seemed poised for strength, like with more moderate Republicans. Rubio worked in 2013 with a bi-partisan group, which included Republican senators Lindsey Graham and John McCain, to co-write the immigration bill that creates a pathway to citizenship for undocumented workers. The Florida senator served as the de-facto spokesman for the bill to conservatives like Rush Limbaugh, who view Graham and McCain as too liberal.

But his colleagues have not returned the favor. Graham has publicly suggested Rubio is too inexperienced to run for president, and he is launching his own longshot campaign, with McCain’s encouragement.