Republican presidential nominee front-runner Donald Trump has called Mitt Romney's attack speech "irrelevant".
Speaking at a rally in Maine, the outspoken billionaire dismissed Mr Romney's earlier remarks with mockery, saying the 2012 nominee was a "failed candidate".
"[Mitt Romney] failed badly" Mr Trump said, speaking of the 2012 campaign to run against Democrat incumbent president, Barack Obama.
Trump reminded the crowd that he had endorsed and funded Romney four years ago. "I backed Mitt Romney. I backed him - you can see how loyal he is - he was begging for my endorsement. I could've said 'Mitt, drop to your knees' and he would've dropped to his knees. He was begging."
Trump also claimed that Romney didn't run in this campaign because he was afraid of clashing with his former benefactor.