At the 2015 Values Voters Summit on Friday, Republican presidential candidate Donald Trump was booed when he called fellow candidate Senator Marco Rubio a "clown."

"You have this clown Marco Rubio," said Trump, and the crowd's immediate gasps and groans built quickly into a round of booing. "I've been so nice to him," Trump said to try and recover.

Before and after the Rubio moment, Trump was enjoying his usual rounds of applause for the same lines he typically receives them for, such as mocking John Kerry riding a bike during the Iran deal negotiations.

Rubio spoke to same crowd earlier Friday, also receiving many applause lines for his stump speech, and according to attendees a few ovations, including when he had the opportunity to announce to the crowd that John Boehner was resigning as Speaker of the House.

The typically bulletproof Trump has seen some weakness since the second debate. The chink in his armor after bashing Carly Fiorina has left him more open to criticism, and not just from his fellow candidates.

Trump was also booed earlier this week during the Pope's visit.

Donald Trump is usually shown on television in front of rapt crowds who are there to support him. At the Values Voters summit we see how he handles a crowd that he doesn't know as well as he thinks, and who appear to know him a little better than he'd hoped.