Trump 'fundamentally breaks' the GOP: Ex-Bush 43 aide

The Republican Party needs candidates to stay in the presidential race in order to force a brokered convention and deny front-runner Donald Trump the nomination, a former senior aide to President George W. Bush said Friday.

That strategy will surely alienate some of Trump's supporters, said Sara Fagen, now a partner at public affairs firm DDC. But while Texas Sen. Ted Cruz, who trails Trump in the delegate count, would have a tough time against Hillary Clinton in a general election, he would keep the core of the party together, she added.

"Donald Trump fundamentally breaks the Republican Party, and it is not the same party moving forward ever again," she told CNBC's "Squawk Box." "That is a worse prospect for most Republicans than Ted Cruz or a weakened Marco Rubio or some other scenario."

Fagen said it is possible but unlikely that a Republican with "enormous stature" like 2012 GOP nominee Mitt Romney or House Speaker Paul Ryan would be put forward if Trump enters the Republican National Convention without the required number of delegates to lock up the nomination.

A more likely possibility is that a Republican governor steps up and runs on a third party ticket, she said. "I think there's a scenario where if he's the nominee, one of these governors looks at the race and says, 'Why not? Why not run as an independent? This is not my Republican Party anymore.'"

Fagen noted that the delegates at the Republican National Convention are made up of the establishment's base, so they may not side with Trump in a brokered convention.

There is an argument to be made that if Trump does not secure the required delegates going into the convention, he is on equal footing with all other candidates, regardless of his technical lead in the delegate count, said Rick Tyler, former senior communications advisor for the Cruz campaign.

"Remember, a lot of the delegates who are elected … to go to the convention, most of these people have known the party rules for long time. They live to go to the convention. They have run to go to the convention. They may or may not be Donald Trump supporters," he told "Squawk Box."

To be sure, many people will be upset if Trump maintains his lead but is not nominated, Tyler said. It remains an open question whether Trump supporters will vote against the GOP, sit out the election, or settle on the party's alternative, he added.

Cruz asked for Tyler's resignation last month after the former aide admitted to promoting an inaccurate story about Rubio on social media.