"Trump is not a strong general election candidate. He's showing surprising strength in the Republican primaries. But that's only 20 million people," said Murphy, who ran the $100 million Jeb Bush super PAC Right to Rise. "The regular election is 128 to 129 million people; different planet, not Trump friendly."
Murphy told CNBC's "Squawk Box" he would not support for Trump, even if the billionaire were to get the GOP nomination. "Personally, I'd never vote for Donald Trump. I'll write in somebody better. But that's just me. The party will support him if he's the nominee."
Acknowledging the revolt among grass-roots conservatives, Murphy said Trump won't unify the fractured GOP. "We're going to have to rebuild it. We've got problems. But Trump is not the answer. He's poison." Murphy has also advised Arizona Sen. John McCain, former California Gov. Arnold Schwarzenegger, and 2012 GOP presidential nominee Mitt Romney
Like former candidate Bush, who on Wednesday endorsed Ted Cruz, Murphy said the Texas senator stands the best chance of beating Trump for the nomination. But Murphy said Cruz would also have difficulty beating Clinton.
Trump and Cruz split Tuesday's nominating contests. Trump won 58 delegates in the winner-take-all Arizona primary, while Cruz won all 40 delegates from the Utah caucuses.
In the primaries and caucuses so far, Trump has secured 738 delegates to Cruz's 463. Ohio Gov.John Kasich, coming off his home state win last week, was not able Tuesday to add any delegates to his total of 143.