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Trump? Carson? Here's who should drop out

With just over two weeks to go until actual voting begins, there are very few opportunities for candidates to shift the trajectory of the race. Thursday night's debate was one of them. While I don't think much will change after last night's performance, we did learn a few things... Here are my takeaways.

1. Donald Trump is a good candidate.

Donald Trump is really good — good at politics. Now to be clear, I definitely don't think he's qualified to serve as president. And, I'm not saying he'd be a great nominee for the GOP. In fact, his nomination would be a disaster for the Republican Party. But, he's very smart, very skilled, and he's much better at politics than most people give him credit for. When it comes to message discipline, he's easily one of the best ever. This is why there's a small but growing chorus of smart people who think he could beat Hillary Clinton in a general election.

2. The Cruz Rubio fight is going to get really nasty.

While Senator Ted Cruz took incoming from several candidates, the Rubio hits seemed especially tough and appeared to really annoy Cruz. These two obviously know each other from the Senate and it seems as if neither can understand why the other is standing on stage. Watch for this conflict to heat up even more than is obvious today.

3. The 'establishment' should never have gotten rid of all the straw polls.

Nothing occurred at last night's debate that will cause the race to fundamentally alter course. And, in this contest, beyond money (which has mattered little in this race), debates are the only yardstick we've had to measure the candidates' success. This has been bad for establishment candidates. They killed the straw polls, a metric that could have helped a few of them. Cruz has benefited most from the intense focus on these debates.

4. Fox News was right to exclude Senator Rand Paul the debate stage.

Thursday night's debate was easily the most substantive debate of the cycle and the primary reason was that there were fewer candidates on the stage. The Republican National Committee and future debate hosts should ensure there are no more than six candidates on the main stage. And, for that matter, they should end the undercard debates. They made sense early on. But, at this point in the cycle, they are a waste of everyone's time.

5. Ben Carson should exit the race.

Ben Carson continues to lack the substance necessary to wage an effective general election campaign and to serve as president. Last night, was important for him and he blew an opportunity to prove he belongs on that stage. And, finally, his campaign appears to be in total disarray, with more staff departing by the day. One can only conclude that his presence is hurting the Republican Party's chance to nominate the best candidate. His 9 or 10 percent of the vote would do a lot to boost Jeb Bush, Marco Rubio or one of the other mainstream governors.

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.

Follow CNBC's Opinion section on Twitter @CNBCopinion.