Romney also has various unpledged delegates and there are super-delegates.
However, basically if Santorum, Gingrich and Ron Paul can combine to contain Romney to less than 47% of the delegates in the remaining primaries then there would be brokered convention.
Gingrich and Santorum need some more timely surges in support and be able to dominate more in southern states.
60% that Obama wins a second term
63% Republican majority in the House
59% Republican majority in the Senate
10% that there will be no party majority in the Senate (a few independents and the VP would hold control)