Donald Trump would lose handily to Hillary Clinton in a general election for one key reason, former Bush 43 aide Tony Fratto said Thursday. "In the very best of years, when you have a unified GOP, Republicans have an electoral disadvantage relative to the Democratic candidate; that's pretty clear," Fratto told CNBC's "Squawk Box."

"Trump is not going to bring unity to the party. If it's a contested convention, and it's someone else, that's not going to resolve unity either. So, [with] a divided GOP and a fairly unified Democratic candidate, I don't think this is a hard question," said Fratto, the former deputy White House press secretary and now managing director at consulting firm Hamilton Place Strategies.

Clinton notched victories in Ohio and Florida on Tuesday, two key swing states. Trump won the Florida GOP primary, but lost in Ohio to the state's governor, John Kasich.

Trump has accumulated 683 delegates, while Texas Sen. Ted Cruz is in second place with 422, according to NBC News. A GOP candidate needs 1,237 total delegates to secure the nomination. The business mogul might fall short of the majority required, enabling the party's establishment to put forward another name at the July convention in Cleveland to formally pick its candidate.

On Wednesday, Trump warned of riots if he is denied the party's presidential nomination after a string of primary election victories, raising the temperature even more in the heated GOP race.

"If we just look at polls, Cruz is at least running even with Secretary Clinton. That's at least a competitive race. Trump is not running very close at all, but I think the difficulty is what does it look like, whether it's Cruz, Kasich, Trump, after a GOP convention, and what's going to be a contested GOP convention," Fratto said.

According to recent polling data, Cruz and Clinton were in a virtual tie, while Clinton held a sizable lead against Trump. Florida Gov. Rick Scott, who in the wake of Trump's Florida victory said fellow Republicans should coalesce around the front-runner, told "Squawk Box" he could relate to the real estate tycoon.