All the other candidates know that if Cruz loses Iowa, the crown jewel of his campaign, he will collapse heading into NH and SC where he is not even second. Cruz has put all his chips on Iowa. It remains to be seen how it turns out.
IMO Cruz made a huge mistake going after Trump this early. The two of them could have made it a two person race after the first four primaries. Instead, Cruz launched his attack now and invited the street fighter from Queens to pummel him hurting Cruz in the polls and providing an avenue for the GOPe candidate, namely Rubio to move into second place. When that happens, the GOPe will coalesce behind Rubio, which could force a brokered convention.
Here is why I (respectfully) say you are wrong. Rubio and Paul do not benefit from trying to take votes away from Cruz. The big opportunity tonight is to steal take away Trump supporters. Trump has the biggest piece of the pie. Cruz supporters aren’t going anywhere.
Also, it will be interesting to see Jeb go after Rubio. Bush PACs are spending $1 million dollars per day ripping Rubio. It has had little effect. It is relentless.