The beginning of the end of Donald Trump?

While it's true Donald Trump's political demise has been written before and he has continued to defy political pundits with his strength atop the polls, we will look back at last night's debate and see it as the beginning of the end of Donald Trump's grip atop the GOP field. Three reasons make this so.

First, the petty insults about people's appearances and intelligence levels are getting old. Sure, it was entertaining for a few months while we all watched in wonderment of how someone in such a prestigious position could say such outrageous statements. But, everyone in show business knows you need a second act. What was funny and bold is now starting to become boring. How many times can one person insult Rand Paul? It's just no longer interesting. Most Americans who follow politics only peripherally will be slower to come to this conclusion, but they will eventually get there.

Second, as last night's debate moved into substantive discussions, Donald Trump faded in the background. He's no doubt a highly intelligent person (just ask him!), but his command of policy is weak and he doesn't seem to have improved or gained new knowledge since entering the race. He's not in command of facts and figures and his answer simply is that he'll hire the best people. The best people?? Seriously? That's a weak answer. It might be an acceptable band-aid of a statement in the early summer months of a campaign, but it won't fly long-term. He has one, maybe two more debates to show he has a deep command of issues.

The challenge for Donald is the narrative on him is starting to shift. He's no longer going to get away with petty insults and mistakes about foreign policy. The political press is becoming more aggressive, as are his opponents. Donald Trump is easily irritated by tough questions. He showed that again last night. And when he's irritated, he more likely to make mistakes. Seasoned candidates understand this dynamic. They understand there's no second take on live television. It's only a matter of time before there's a moment on camera that even the Donald wishes he could take back.

That's not to say Donald Trump will fall immediately. He has a strong base of support that appreciates his blunt style and some of them will stick with him no matter what he says or does. He appeals to peoples' fears, which won't fade overnight. He will be a factor in the race and he may play a pivotal role in deciding who will carry the party's mantle. Like it or not, the Republican Party needs to take him seriously.

Third, there's no evidence Donald Trump is building the infrastructure necessary to win the GOP nomination. Since 1980, the single biggest predictor of who will win the GOP nomination is the number of endorsements from political leaders – both past and present. Right now, former Florida Governor Jeb Bush leads that contest followed by Governor Chris Christie. While this might seem like an overly simplistic metric, it's significant because all these officials have political capital and organizations – organizations that can be a lifeline in a long primary fight. They need that support from local leaders when they're trying to get on the ballot in all 50 states (which can make even the best lawyer's head spin). I'm unaware of a single endorsement Mr. Trump has garnered from a senior Republican. And, without the depth of a political organization, even the most knowledgeable and tempered candidates fail.

Right now, it's unclear what Mr. Trump's second act is beyond more rude comments. He lacks the substance to be president and he lacks the political organization to win a long primary contest.

Commentary by Sara Taylor Fagen, a partner at DDC Advocacy and a former Political Director for President George W. Bush. She is also a CNBC contributor. Follow her on Twitter @sarafagen2.