I’m talking Romney’s campaign in the primary, not the general.
People around the GOP have admitted they accidentally made a path for Trump. The path was mapped out so the candidate with the most money would win easier. They thought it would be Bush.
Social conservatives have soured on Trump to some extent, and they were the Achilles’ heel for Romney as well in the primary. They ended up going to Gingrich and Santorum because they didn’t trust Romney’s stances on abortion and same-sex marriage. Trump his similarly flip-flopped and is seen as more untrustworthy than most candidates. Romney had an advantage of having Mormon support though which Trump doesn’t have.
Trump and Romney are also northeastern Republicans. So culturally they were unbeatable in NH and the northeastern states. The GOP also set up the primary to have more winner-take-all contests in those blue states vs. the southern states, so that will help Trump fend off an upstart from a more reliably conservative challenger like Cruz.
The major battle might be for the South. Cruz absolutely needs to win those to have a chance. If Trump does well in Southern states there might be no stopping him. Obviously Iowa is not enough and was not enough for Huckabee and Santorum.
The GOPe set up Florida as a winner take all state betting on Bush or Rubio. Florida is going Trump, like it or lump it.