Donald Trump is constantly repeating 2 things about Hillary Clinton

Republican presidential front-runner Donald Trump says the same two things about former Secretary of State Hillary Clinton in almost every single campaign speech these days.

His favorite zinger is that Clinton, the Democratic front-runner, doesn't have the "strength or the stamina" for the Oval Office.

"One thing with Hillary, she doesn't have the strength or the stamina to be president. She doesn't have it," Trump said at a Wednesday-night campaign rally in Manassas, Virginia.

"You ever see Hillary? And I said, 'She doesn't have the strength, she doesn't have the stamina,'" Trump later said.

Trump's other lines that Clinton shouldn't even be "allowed" to run for president due to her controversial email practices at the State Department. The FBI has said it is investigating whether any material was mishandled in connection to Clinton's email account, which was run out of a private server in her home.

"Hillary shouldn't be allowed to run because what she did is illegal. What she did is illegal," Trump asserted Thursday.

Trump is fond of repeating the same zingers against his presidential rivals, hoping they'll stick.

He bragged Thursday that he had the "ultimate sound bite" against Sen. Marco Rubio (R-Florida) after criticizing him for his relatively poor Senate-attendance record. And for a while, Trump constantly repeated a claim that retired neurosurgeon Ben Carson never recovered from the "pathological temper" he said he had as a young teen.

Trump's most famous one-liner, however, was against former Florida Gov. Jeb Bush (R), whom he relentlessly called a "low-energy person." That insult caught on so much that it became an inside joke among politicos, and Bush himself referenced it during a debate.

The real-estate mogul told Business Insider last month that the "low-energy" line simply came to him while observing Bush.

"Yeah, that's once in a decade," Trump said of the insult's success.