Winning two primaries does not make him the Republican party candidate. The idea is that while Trump has 40% at the moment that means he has more than any other candidate but as time goes on other candidates drop out and the 60% that don't vote for Trump at the moment still won't vote for Trump but for somebody else.
Apart from that other people start to understand Trump and how to respond to him as time goes on and they think that as time goes on the establishment candidate Marco Rubio will benefit from that.
This is plausible, thanks. Followup question for the first part - why do bookmakers favor Rubio over Cruz? Cruz has advantage in polls and IA (30× difference between their odds), but disadvantage in NH (4× difference). I could see "non-Trump voters will tend to go to anyone except Trump when their candidate drops out", but "will tend to go to Rubio" seems rather more specific.
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