Being as it's a lefty website, the Daily Kos wasn't threatening to run a 3rd party bid for President. It is just plain silly for Republican candidates to attend a debate with a moderator who is actively discussing running against them. Additionally, the setup smacks too much of his realityTV show where he judges their performance and picks a winner (his endorsement). It's just a bad idea and most Republican candidates are going to avoid it. I wouldn't be surprised if the thing is just canceled for lack of participation.
The people who are criticizing the debate don’t want them to do it, because of the birther issue. Trump isn’t going to run.