After watching the debate i think he made a HUGE mistake now not showing up.
I disagree. I think that those who already support his candidacy will support his decision not to participate, for exactly the reason he stated. Those who are capable of observing that he is manifestly unsuited to the office of the presidency have already done so, long ago. For other candidates, his absence seems to have made a difference, but for the Trump campaign it really doesn't matter.
I also don't think you can make inferences from his performance next week. He can poll 30%, but still only get 20% and it may have nothing to do with this debate or any other. For one reason, his supporters are among the most irregular of voters. At one point he was polling at 40%, but among that share, only about a quarter of them are estimated to have voted in the most recent general election, let alone a party primary, where the turnout is historically even lower. On the other hand, because it's irregular, the turnout is unpredictable. Maybe he gives them a raging hard-on in a way that no other republican ever did. Like Torie says, we'll know after Monday, which really is the only poll that counts.
I agree with Angus: Trump's not showing up for the debate isn't going to change anybody's mind at this point. Having watched both the debate and Trump's alternative veteran thing, I think Trump did well by staying out of the debate.
That said, I'm not sure if Trump's supporters can be categorized as among the most "irregular" of voters. I think that it's true that Trump supporters perhaps more often than not sit out these elections because traditional (aka "establishment") candidates don't speak to their concerns. Say what you will, Trump and Sanders have managed to tap into the anger/unrest felt by many of today's voters. On Monday, we'll see the first signs of how deep this goes.
