Andy Card, who served as chief of staff to President George W. Bush, said on "CBS This Morning" Friday that the idea of Donald Trump as president worries him, citing the billionaire's "emotional" reactions as cause for concern.
"Yes, he scared me," Card said. "I will admit, when Donald Trump entered the race, I didn't think he had a chance."
"I want him to succeed, but he's gotta make the move to show me that he's worthy of my support, worthy of my vote," he added. "I will vote for a candidate -- right now I will probably write in a name, but I will vote. But I want him to really pay attention to the responsibility of being a president. Presidents do not have the luxury of making easy decisions. They make brutal, tough decisions."
Card, now the president of Franklin Pierce University in New Hampshire, was also an adviser to President Ronald Reagan and President George H.W. Bush. Card was the White House chief of staff on Sept. 11, 2001, and headed the younger Mr. Bush's White House Iraq Group.
Joining other prominent Republicans who have said they could not back Trump, both Bush presidents said earlier this week that they would not endorse the Manhattan business mogul in his bid for the Oval Office.
The former White House adviser told the "CBS This Morning" co-hosts that Trump would have to change his behavior toward the Republican Party if he wants to win in a general election.
"He's gotta demonstrate that he cares about everybody in the party," Card said. "He has to change how he will be perceived to do the job. It's not all about Donald Trump now."
Of some Trump policies, Card dismissed them, saying "he's just giving a quick response."
"He's not tasting his words before he spits them out, and there are consequences to that," he continued. "He should actually think, how will these words be heard? Will they allow me to build a bridge so that I can earn respect and earn support?"
Card said he was "not sure" that Trump has completely converted to a "listener," adding that the billionaire, now the Republicans' presumptive presidential nominee, was "probably not" interested in hearing advice from experts and other former government officials.
Still, Card said, "I hope that he will find the courage to acknowledge that he needs advice and listen to it."