I'll disagree you can't get the nomination if you are pro-life or pro-2nd-amendment democrat. Obama basically was. And really gun control has always been pretty low on the rung of importance for democrats in the grand scheme of things. What you can't do is run as a pro-lifer who is going to open up that can of worms in congress, the senate or courts.
Anyways...
Good article on the path to 1237 delegates a GOP nominee needs from 538. And really why a big lead by Trump and/or Cruz early may not mean anything. The GOP establishment has intentionally neutered those states as much as possible to avoid the outside candidates getting control of the race through proportional delegate awards. For the establishment and trustworthy states it's winner take all.
It is a three horse race between Trump, Cruz and Rubio at this point. Any establishment GOP are going to support Rubio but need to do so early on in the delegate battle (may also be able to substitute any of the other establishment guys like Christie, Bush or Kasich...the last one being the best bet to completely win over independents and dissatisfied democrats).
http://fivethirtyeight.com/features/...rump-and-cruz/