538 has this to say:
The soonest that Mr. Romney could officially clinch the nomination is May 22, when Arkansas and Kentucky vote. That situation would require Mr. Romney to win at least 95 percent of the delegates in Arkansas, Kentucky, Louisiana, Wisconsin, Maryland, Connecticut, Delaware, New York, Pennsylvania, Rhode Island, Indiana, North Carolina, West Virginia, Oregon and the District of Columbia, and to receive endorsements by virtually all of the Republican Party’s 77 undecided superdelegates by that time.
Some of those states, of course, are not so strong for Mr. Romney. And even if he won 70 percent of the delegates in those states, as well as in Texas, which votes on May 29, he would still need to wait until June 5 — when California and New Jersey vote — to clinch the nomination.
Later, "As this becomes increasingly clear to voters, they may come to see their choice as being less one between Mr. Romney and Mr. Santorum and more one between Mr. Romney and a brokered convention."