Former President George W. Bush’s chief of staff says that the prospect of a Donald TrumpDonald John TrumpHouse Democrat slams Donald Trump Jr. for ‘serious case of amnesia’ after testimony Skier Lindsey Vonn: I don’t want to represent Trump at Olympics Poll: 4 in 10 Republicans think senior Trump advisers had improper dealings with Russia MORE presidency frightens him.

“I will admit, when Donald Trump entered the race, I didn’t think he had a chance,” Andy Card said on “CBS This Morning." "When he started to gain traction, I became a cynic. I wasn’t sure he could make it. I was saying, ‘sane people will recognize this is not going to happen.’ ”

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“Well, it happened. He scared me. I want him to succeed, but he’s got to show me he’s worthy of my support, worthy of my vote.”

Card said that Trump’s blunt rhetoric concerns him, adding that the presumptive GOP presidential nominee should measure his words more carefully.

“He’s not tasting his words before he spits them out,” he said. "There are consequences to that. You should actually think, ‘how will these words be heard?’

“I want him to really pay attention the responsibility of being president. Presidents do not have the luxury of making easy decisions. They make brutally tough decisions.”

Card added that Trump has a steep climb before convincing establishment Republicans that he respects them.

“Now he’s the leader of the Republican Party, for all practical purposes,” he said. "He’s got to demonstrate that that he cares about everybody in the party, that he helps the party win.

“I hope that he will find the courage to acknowledge that he needs advice,” Card added. "You don’t command respect. You have to earn respect.

“I take the vote for president very, very seriously. Right now I will probably write in a name. I want to find out what it will be like, Donald Trump, when you have an impossibly tough decision where there is no measure of popularity."

Former Presidents George H.W. Bush and George W. Bush plan to skip the Republican National Convention in July. Trump will likely become the GOP presidential nominee during the event in Cleveland.