Obama has a total of 1,915 delegates overall, including endorsements from superdelegates. Clinton has 1,721, according to the latest AP count.
Obama is a little more than 100 delegates short of the 2,026 needed to clinch the nomination. He cannot make up that ground in the Kentucky and Oregon primaries because of the proportional way in which Democrats award delegates. The two states have a total of 103 delegates at stake Tuesday.
But winning a majority of pledged delegates could help his case with undecided superdelegates, who are free to support whomever they choose at the party's national convention in August. Obama argues that superdelegates should support the candidate who wins the most delegates in the primaries and caucuses. Otherwise, he says, they would be overturning the will of the voters.
Obama, who overtook Clinton in superdelegate endorsements a little more than a week ago, picked up six more Monday. Clinton added none.
Clinton argues that superdelegates should exercise independent judgment.
Her campaign is also trying to change the math by getting the delegates seated from the Michigan and Florida primaries. Clinton won both primaries, but the states had been stripped of their delegates for violating party rules by holding their primaries before Feb. 5.
The Democratic National Committee's rules panel is scheduled to address the issue May 31. If any of the delegates are reinstated, it would increase the number needed to clinch the nomination.