"Anybody who says they think they know what he's going to be [like as president], they don't know what they are talking about because he will change what he's going to be," Fratto told CNBC's "Squawk Box."
Fratto said a general election featuring Trump against Democrat Hillary Clinton does not bode well for his party. "Republicans will lose to Hillary Clinton if Donald Trump is the nominee. And I don't think that's even a hard question."
In the latest polls, aggregated by RealCearl Politics, Clinton would edge Trump with 45.4 percent support to 42 percent.
If Trump were to become the GOP nominee, Fratto said, he would not support the billionaire businessman. "I could not in good conscience help this man become president of the United States. I have to live with myself after that. I don't want him representing me. I don't want him representing my children. I don't want him representing this party."
Fratto had supported the candidacy of Jeb Bush before the former Florida governor dropped out of the race. He has since thrown his support behind Rubio. "There are other candidates who have a chance right now. I'm not a big Cruz guy. But this is still a race, and Cruz is still close by."
In another "Squawk Box" interview, Kay Bailey Hutchison, a former Republican senator from Texas, gave credit to Trump for mounting an unexpectedly strong bid. But she said the real estate mogul's lack of foreign policy experience and controversial statements should raise red flags for voters.
"[Trump] has certainly run an incredible campaign that nobody would ever have thought. It's [also] troubling some of the things he's said," Hutchison said, pointing to concern expressed by two former GOP presidential nominees, Mitt Romney and Arizona Sen. John McCain. "We need to be thinking about a presidential candidate that's presidential who can deal in foreign policy."
Following Romney's scathing anti-Trump speech on Thursday, McCain, chairman of the Senate Armed Services Committee, said he shared the concerns of the former Massachusetts governor. In a statement, McCain called Trump's views on foreign policy "uninformed" and "dangerous."
"I admire what Mitt Romney has done. It was very difficult, I'm sure for him to jump back in and say, 'Are you really thinking this through,'" Hutchison said. "I think people will listen to many of the people who are not supporting Donald Trump. And I think they're listening to the people who do support him. And that's what a primary is for."
Heading into four contests on Tuesday, Trump has a double-digit lead in Michigan, the most delegate-rich state among the four, according to a new NBC News/Wall Street Journal/Marist poll. Idaho and Mississippi also hold primaries, while Hawaii holds caucuses.
"[Trump] has played by the rules. He is winning," said Hutchison, who has yet to endorse any of the Republican candidates. But with Trump holding only about a third of the 1,237 delegates needed to grab the GOP nomination outright, Hutchison said: "The race is not over. And people are now speaking about all the factors that they want in a president. There may be a different outcome."
If Trump were to win the nomination, Hutchison won't say whether she's stand behind her party's candidate. "I want to see what he does. I think it's a little too early to tell. He is beginning to think this is a reality. Is that going to make a difference? I don't want to make a commitment right now. Let's see how this plays out."