Chuck Todd asks a very interesting question this morning: Were Saturday's contests because of closed primaries or because Trump is losing momentum? Going into Saturday, people speculated that the closed primary system would be disastrous for Donald Trump, yet he still won two of those states. What we did learn from those states, however, is that the answer to Todd's question is... yes.
Thursday's debate absolutely was disastrous for Trump - it is the only major event* that happened between early voting like states in Louisiana and Saturday's primary day. However, it wasn't disastrous in the way you think. People loyal to Trump will not change their vote because he stuttered, contradicted himself, or defended the size of his penis on national television. People who leaned Trump switched their vote to Cruz, but because Cruz hit high on Trump and didn't hit him low like Rubio was doing, Cruz picked up Kasich and Rubio supporters.
The closed primary allowed those supporters to defect to Cruz, who appeared strongest after Thursday. It kept out the Democratic crossover voters. Trump's momentum hit a hiccup after that debate and after this weekend, and it may have stifled him in Mississippi, Idaho, Hawaii, and Michigan. It is today's primaries that will determine the future of this race.
If Trump loses his lead in any of those states or, by the grace of God Almighty, loses any (or all four!) of them outright, it means that, yes, Trump's momentum is lost. He can regain it going into more liberal-non southern states, but if Cruz overperforms in Michigan**, then Trump will begin sweating. And a sweating Trump creates more incidents like Thursday night's debate. And the Republic still stands.
*Romney was a non-factor. It played into the debate a bit, but it was during the day while the debate was a prime time event.
**Kasich is a thing that will happen in Michigan. But I try not to think about it, and neither should you.