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. Some of Mr. Santorum’s supporters may desert him once they view the race in those terms, making Mr. Romney’s path easier still.

And if the nomination were to go to the convention floor, it is not clear how Mr. Santorum would expect to prevail there. Before Michigan, it was possible to envision a situation in which Mr. Romney would have the plurality of delegates but Mr. Santorum would do better by measures of popular support, like the aggregate popular vote, national polls and the number of states won. Mr. Romney now leads in all those metrics. And he has won in enough key states — including Florida, Michigan, Ohio and now Illinois — that it would be hard for one of his opponents to assert that his nomination had vetoed the will of the Republican electorate.

Before an election in 1983, Gov. Edwin W. Edwards of Louisiana declared, “The only way I can lose this election is if I’m caught in bed with either a dead girl or a live boy.” We’re getting close to the point where it might take a major revelation for Mr. Romney to lose, something that substantially and permanently alters the way voters view him.