Zhang, at current rates at which all the demographics are voting for the candidates, every realistic projection has Romney ending with maybe 1050-1200 delegates. To win at the convention he’d need 1,144. He can get enough from Ron Paul or GOP elite arm-twisting to hit that limit.
Because of Ron Paul, Newt and Rick will likely not have enough delegates between them to combine and get over the 1,144 even if Romney falls short. And it’s very clear that Ron Paul will give his support to Romney, not the other two guys.
The only path to victory I can see is for Rick or Newt to drop out, strongly campaign for the other one and perhaps announce a V.P. team-up. Then it is almost certain the remaining one can get a plurality in most of the increasing number of winner-take-all states, and sometimes 50%. Both of those help them get disproportionate shares of delegates (which Romney has done so far in states like FL and AZ, which is why with only 40% of the vote he has over 50% of the delegates so far).
Ron Paul's people went to McCain's side in 2008, but only after he'd gotten a majority. Without a majority, Romney may not get those delegates, especially if Gingrich and Santorum are still in the game. Romneycare has to be a major sticking point for the libertarians who support Paul.