By April 24th, we will know results of LA, MD, DC, WI, CT, DE, NY, PA & RI. That is a total of 401 delegates.
Romney has 497, Santorum has 183, Newt has 135 Committed delegates according to Wiki.
If Santorum wins 50% of delegates through April 24th, he will have 183+.50x401 =384 delegates.
If Romney wins 30% of delegates through April 24th, he will have 497 + .30x401= 617 delegates.
After April 24th, there are 796 delegates remaining to be won.
Santorum needs to win (1144-384)/796 x 100 = 95.5% of remaining delegates to get to 1144.
Mathematically possible, in reality, impossible.
Romney needs to win (1144-617)/796 x 100 = 66% of remaining delegates. Mathematically possible, in reality improbable.
Conclusion? We are going to a brokered convention. And that convention is highly unlikely to pick a 2nd place winner. So if Romney does not get it, it will be some one else from the sidelines.