Even so, it is unlikely that she will be able to erase Obama's lead among pledged delegates by the end of the primary season. To win, she would need a large margin in so-called superdelegates, party leaders free to vote for whom they choose. But they have been trending Obama's way, and by some tallies he now leads in the superdelegate count.
"In terms of numbers, it's over," Professor Sabato says.
Campaigning in Oregon on May 10, Obama barely mentioned his Democratic rival. Instead, he began attempting to draw contrasts between himself and Senator McCain, saying that the Republican had received a "free pass" for weeks as the Democrats slugged it out in primaries across the country.
Obama said he would be open to meeting McCain in unscripted town-hall events. But in a preview of how hard-hitting such events might become, he said he might raise such issues as McCain's connection to the 1987 Keating Five savings-and-loan scandal. He also called McCain's proposal for a summer moratorium on the federal gasoline tax a "pander" that belies the Arizona senator's straight-talk image. "He didn't even have a way of paying for it," said Obama of the gas-tax proposal.
On May 10, the Obama campaign also announced the beginning of what it said would be a 50-state voter-registration drive in advance of the November elections.
That is the sort of unifying gesture that Obama needs to make if he indeed becomes the nominee, says Allan Lichtman, a political history professor at American University in Washington. With the Republican nominee burdened by association with an unpopular war, economic woes, and an incumbent with high disapproval ratings, "only a failure to unify can derail the Democratic ticket this year," Professor Lichtman says.