Wheels coming off the Trump campaign: Fagen

Donald Trump's defeat in the Wisconsin GOP primary was a turning point that could derail the business magnate's campaign, Bush 43 political director Sara Fagen said Wednesday.

"He is a sore loser, and he has proven it time and time again. This is a person who has not built a campaign infrastructure and now the rubber hits the road in these delegate contests. He's pointing fingers, he's lashing out perhaps even more so than he's done in the past," Fagen told CNBC's "Squawk on the Street."

"Last night I think was a real turning point in the Republican race. Donald Trump can now, because of his devastating lost last night, win every single delegate in New York and most of the delegates in the New Jersey and Pennsylvania and many of the delegates in the upcoming contests, and still not have enough delegates on June 7, the last day of voting," added Fagen, who is now a partner with the Washington consultancy firm DDC Advocacy. "I think it is now inevitable that this will be an open convention."

Sen. Ted Cruz of Texas handily defeated Trump in Wisconsin on Tuesday, but the billionaire businessman is still leading with 753 delegates to Cruz's 514 in the race for the 1,237 needed for the nomination.

"This is clearly not resolved. It appears to me it's is going to go to a convention that is open. I don't see how Trump gets to the number before the convention," Republican former New Hampshire Gov. Judd Gregg told CNBC's "Squawk Box." "If [Trump's] within 100 delegates of the nomination, then he'll probably be nominated at the convention. If he's 150, 300 delegates away, I think this convention becomes very unpredictable."

"The most interesting number that came out of Wisconsin [was that] 30 percent of Republicans in the primary said they would not vote for Trump in the general, 30 percent said they would not vote for Cruz in the general. You cannot be elected president if you can't maintain the base, and those numbers are pretty staggering," said Gregg, who also served in the Senate.

Trump's campaign had been steamrolling until recently, as the GOP front-runner took heat for remarks made on abortion, torture and the roll of nuclear weapons in international politics.

Vin Weber, a partner at Mercury and former Mitt Romney advisor, said Trump's campaign has lost its steam.

"A few weeks, Donald Trump was on a roll. It looked like he would get close to, if not over, the 1,237 number and a lot of the Republican leadership was thinking 'OK, how do we get comfortable with this? He's a dealmaker, he's a negotiator, maybe we can get comfortable with him,'" Weber said on "Squawk Box." "I think this momentum to get comfortable with Donald Trump was stopped cold last night."

Wisconsin also held a Democratic primary, which was won by Sen. Bernie Sanders of Vermont. Sanders' win also marked his sixth straight victory over Hillary Clinton, the former secretary of state.

This string of victories may lead to a significant shake-up in the Democratic race, Gregg said.

"I wouldn't be surprised if the Democratic party gets thrown into some disarray here, and we saw Joe Biden come out the back room because of the fact that the party decides that Hillary [Clinton] is just too weak after being beaten by Sanders everywhere she's going right now," he said.

The Wisconsin primary did not carry a large sum of delegates, but boosted Sanders' momentum as the April 19 New York primary approaches.

"Momentum is that within the last couple of weeks, there have been national polls which have had us one point up or one point down," Sanders said in his victory speech Tuesday.

Still, in the race for the 2,383 delegates needed for the Democratic nomination, Clinton leads Sanders by 1,728 to 1,058, according to NBC News.

"The math doesn't really change," Steve McMahon, co-founder of Purple Strategies, said in another "Squawk Box" interview. "Sanders … only got three delegates more than Hillary Clinton, so this thing is effectively by the math."