Critics are now using Donald Trump's favorite Jeb Bush burn against him

Republican presidential front-runner Donald Trump is now getting stung by his favorite criticism of rival and former Florida Gov. Jeb Bush (R).

After the lengthy, three-hour CNN debate, multiple reports on the event - and even the Bush campaign - declared that Trump was, ironically, the "low-energy" candidate on the stage.

"Trump was, well, low-energy," Politico's Glenn Thrush wrote for his first takeaway on the debate.

"As usual, he didn't let a single attack go undefended," Thrush wrote. "But as opponents like Bush and Rubio gained confidence and energy as the marathon debate dragged on, Trump slumped - his voice hoarsened and the pace of his outbursts slowed to a trickle."

Other reports agreed with this assessment. BuzzFeed's Rosie Gray's story on the evening was titled: "Donald Trump's Quieter, Low Energy Night." Jim Newell's piece in Slate was titled: "When Donald Trump Became the Low-Energy Candidate." 

To be fair, Trump was hardly the only candidate to lose some steam during the debate's long slog - especially in the final, third hour.

At the end of the debate, several of the candidates seemed to phone in their answers to questions about what their Secret Service code name might be and which woman should be on US currency notes. Trump and two others suggested their immediate relatives for the $10 bill.

However, there was clearly some gleeful criticism of Trump, who has so frequently bashed Bush for being a "low-energy person" that it's become an inside joke both on the debate stage and among debate observers.

Kevin Madden, a former aide to 2012 GOP nominee Mitt Romney, tweeted during the debate that the public was witnessing a "Low energy Donald Trump." Others agreed.

When Bush attacked Trump at the start of the event, Trump offered a backhanded compliment to Bush's "energy" in an apparent attempt to get under Bush's skin. After Bush referenced a battery brand for his hypothetical Secret Service code name, he told Trump: "It's very high energy, Donald."

And two Bush aides shared tweets snarking on Trump's energy level: