Donald Trump’s newfound commitment to message discipline and restraint showed some cracks on Thursday, with the Republican nominee attacking an African-American pastor who cut off his political speech in a Flint, Michigan, church.

"Well, I was in Flint yesterday and it was a very interesting experience and got unbelievably good treatment from people, I must say, and even in that audience, the treatment was great. But something was up because I noticed she was so nervous when she introduced me," Trump said in a telephone interview with "Fox & Friends," noting that the Bethel United Methodist Church pastor Faith Green Timmons "was so nervous, she was shaking" when she introduced him for his speech.

But on Wednesday night, Trump apparently couldn’t resist, and again questioned Clinton’s stamina during a rally in Canton, Ohio.

"You think this is easy? In this beautiful room that is 122 degrees," Trump said, although the arena was notably chillier than his usual venues. "It is hot and it is always hot when I perform, because the crowds are so big, these rooms were not designed for this kind of crowd.”

"I don't know folks, do you think Hillary could stand up here for an hour?" he asked.