If Trump loses Iowa, does he win or lose New Hampshire?

[Why]

As tedbessell says above it depends on how he loses.

He he does compared to how he has polling will give us great insight into where things really stand.

If he loses in Iowa but does quite well, or even better, compared to the polling in the lead up to the caucas' and he is still comfortably leading the polling in New Hampshire then he would be expected still to win New Hampshire.

However if he significantly under performs the polls in Iowa then it would give reason to doubt whether his popularity in polling is going to be matched by popularity in voting. Of course the differences in structure between Iowa and New Hampshire might help him. The negative publicity that a bad loss would lead too might hurt him in New Hampshire as well.

If Cruz gains momentum out of Iowa then he might be helped in New Hampshire but not by very much given the difference in demographics and also from a lower starting point. Trump might be hurt but just hang on in a vote that is closely split between Trump, Cruz, Rubio, Christie and perhaps other candidates.

[Why]

I doubt he loses it. It depends what happens in the next month but Iowa is really useless on the Republican side. It hasn't picked a nominee correctly since 2000.

Well, that just means it only failed two times in 2008 and 2012 (barely). Two in a row does not a pattern make.

Also if Trump loses both Iowa and New Hampshire it will quite possibly too two different opponents. Cruz in Iowa, Rubio or Christie in New Hampshire. Then he would probably lose South Carolina to Cruz and Nevada is really up in the air.
This scenario would knock Trump not down to 2nd but to 3rd.

However I still think Trump will go close if not win Iowa and win New Hampshire, South Carolina and Nevada.