"My guess is if Trump can get to over 1,200 [delegates], he'll find the next 30," Murphy told David Axelrod on "The Axe Files," a podcast produced by CNN and the University of Chicago Institute of Politics. "If he's down around 1,175, I don't know if he can do the 50 or not. We'll find out. I'm not so sure. On a second ballot, I think he'll drop a lot."
Murphy, who earlier this year led the Right to Rise super PAC in support of former Florida Gov. Jeb Bush, said he thinks Trump's main rival Sen. Ted Cruz would likely snatch the nomination if the July convention in Cleveland goes to a second ballot and beyond.
"The natural dynamic of the convention will be more Cruz friendly, ideologically, than anybody else. He'll have the credibility of having run and come in second. He'll have won states. He'll have more natural organic supporters on the floor, I believe. And I think you're going to see some of the establishment folks giving up on the math after maybe giving [John] Kasich a ballot."
Murphy left little doubt that he is pessimistic about the chances of either Trump or Cruz in the general election, but that given the choice, he would prefer the Texas senator emerge as the party's nominee.
"Of the two of them, I'm for Cruz over Trump," he added. "I think a Trump nomination would be a disaster for the country. I think a Cruz nomination would merely be a fiasco for the Republican Party."
Despite Right to Rise raising over $100 million to boost Bush's candidacy, the former Florida governor dropped out of the race in February after a poor showing in South Carolina.
"The thing we didn't see coming was Trump. We also didn't see that the populist, conservative movement wing of the party, the tea party wing, would be large enough to accommodate kind of one and a half candidates: Cruz, Trump. And that Trump would have plenty of appeal to the regulars, just based on style," Murphy said.
"So we were fighting for a relatively smaller share of the vote with Scott Walker and Kasich and Marco Rubio,... [and] a few others. So it was a tough situation. And we always knew we had this gap between the perception of invincibility and the reality of the primary. But we did the honorable thing. You go fight. You try. I don't apologize for any of it."
Murphy, currently a Fellow at the University of Chicago Institute of Politics, also offered no apologies for how the super PAC spent its war chest.
"The truth is I'm proud of how we governed it and we're giving a bunch of money back because we were pretty prudent managers," he said. "We're going to get refund checks out in about three weeks."
To hear the whole interview with Murphy, which also touched on his time at Georgetown as a member of the College Republicans, his love of working in blue and purple states, his time on the McCain campaign's Straight Talk Express in 2000, and more, click on http://podcast.cnn.com
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