"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.
The interesting drama heading into the July convention would be if Trump were to fall 100 or more short of the 1,237 delegates needed to clinch the nomination, Murphy said. "We are playing by the rules, no trophy for second place. If you don't get enough delegates you lose."
Delegates pledged to Trump due to their state primary and caucus results could support Cruz if the nominating process goes beyond the first ballot, Murphy said. "A lot of those Trump state delegates are Cruz people."
"[But] if Trump comes in a little bit short [by] 40 or 50 delegates ... because there are enough unencumbered delegates on that first ballot, he might be able to piece it together," Murphy said.
Democrats experienced the last true brokered presidential convention to go beyond the first ballot in 1952. Republicans came close at their 1976 convention.
Regardless of what happens at this summer's GOP convention, Murphy said he's going to "fight the next fight, and the next one, and the next one."
"I'm a conservative Republican. I'm a Jeb Bush Republican. I'm a Scott Walker Republican," he said. "We lost. I get it. But that doesn't mean I'm going to change my point of view."
Wisconsin Gov. Scott Walker, who ended his GOP bid in September, said he'll decide next week which candidate he intends to vote for in his state's winner-take-all 42-delegate primary on April 5.
"I'm more in line, both in thought and process, with Cruz or Kasich than I would be with Trump," Walker told The Associated Press on Tuesday.