I strongly agree that the only way Romney can be stopped is if either conservative candidate withdraws - and it may already be too late, in which case they, not the voters, gave it to Romney.
May I disagree?
If Santorum quits, his voters in future primaries stay home, advantage Romney. They don't automatically vote for Newt, and vice versa.
Both conservatives staying in continue to draw conservatives to the polls, dilute Romney's vote, and pave the road to a convention fight with 1000 conservatives in the delegations that will reject Willard.