Mitt Romney has decided to call it quits.
Read his speech here
Let's take a look at the math for the Republican race:
The total number of Republican delegates is 2,380
The total number of delegates awarded so far is 1,197
Therefore, there are 1,183 Republican delegates left
Republican candidates need 1,191 delegates to win
John McCain has 714 delegates
Therefore, John McCain needs 477 more delegates, or 40.32% of those remaining.
Mike Huckabee has 181 delegates
Therefore, Mike Huckabee needs 1,010 more delegates, or 85.38% of those remaining.
If all of Mitt Romney's delegates (286) should go to Mike Huckabee, then he will have 467 delegates.
Therefore, Mike would need 724 more delegates, or 61.20% of those remaining.
Ron Paul has 16 delegates.
I'll leave that math as an exercise for the reader