Mike Young sent me this observation on the Republican nomination race:
I think the problem is that if there were on option of Trump or “some candidate other than Trump” then (as Trump gets about 40% of the vote) the “united non-Trump candidate” could conceivably beat Trump. The average split is 40% Trump 30% Cruz 30% Rubio. Cruz and Rubio are desperate for the other to drop out, but they won’t go themselves as they only get one shot at president.
Trump also has another card he could play. Ted Cruz was born in Canada. It is therefore disputable if he was a “natural born citizen” and thus eligible to be president. Trump could challenge this, but presumably doesn’t want to as this would leave the field as Trump / Rubio, thus giving a more united opposition.