REUTERS/Mike Blake
Romney, the 2012 Republican presidential nominee, tweeted that Trump's hesitance to disavow former KKK grand wizard David Duke's endorsement was "disqualifying."
"A disqualifying & disgusting response by @realDonaldTrump to the KKK. His coddling of repugnant bigotry is not in the character of America," Romney tweeted.
Romney joined Trump's presidential rivals, Sen. Marco Rubio (R-Florida) and Sen. Ted Cruz (R-Texas), in denouncing Trump's response to the KKK endorsement.
"Really sad," Cruz wrote on Twitter.
"We cannot be a party that nominates someone who refuses to condemn white supremacists and the Ku Klux Klan," Rubio said Sunday.
For his part, Trump said that he did not disavow Duke's endorsement during a Sunday CNN interview because he was provided with a "bad earpiece" that made it difficult to hear. He also noted that he disavowed Duke's endorsement at a Friday press conference during which he rolled out the endorsement of New Jersey Gov. Chris Christie.
"I was sitting in a house in Florida, with a bad earpiece," Trump told NBC's "Today" show Monday. "I could hardly hear what he's saying. I hear various groups. I don't mind disavowing anyone. I disavowed Duke the day before at a major conference."
Trump and Romney have been increasingly critical of each other in public. Despite Trump's repeated refusals, Romney has pushed the former reality-television star to release his tax returns, alleging a possible "bombshell" contained within them.
"Mitt Romney, who totally blew an election that should have been won and whose tax returns made him look like a fool, is now playing tough guy," Trump fired back in a tweet last week.