“Jeb is easily the most prepared, so he needs to raise that issue. It’s easily the biggest difference between himself and Rubio,” one major Bush donor said. “It’s the biggest difference between any governor and senator. Governors have to make the decisions every day that prepare you to be commander-in-chief on day one.”

On the Rubio side, the message from supporters was similarly enthusiastic.

They say the attacks are evidence Bush is feeling the heat of a Rubio threat.

“I don’t know if he’s desperate, but it’s starting to stink like he is,” said Peter Brown, a top Rubio donor in South Carolina. “He’s on the verge of becoming invisible in this race and all of sudden, look who he’s going after.” 

Rep. Tom Rooney (R-Fla.), who is supporting Rubio for president, said Bush’s portrayal of Rubio as inexperienced won’thold.

“Do you really want to boil it down to who the more tenured politician is in an election cycle when voters have shown no interest in government experience?” he asked.