Though the last Democratic contest is not until June 3, the Illinois senator has increasingly begun to focus his campaign on the general election.
This week he announced plans to visit important swing states Michigan and Florida -- which were stripped of their delegates to the party's convention this August in Denver after they broke party rules by moving their primaries ahead on the calendar.
While Clinton is facing increased pressure to drop out of the race, her senior adviser Terry McAuliffe told ABC News he is "100 percent" certain she will stay in the race until the last primaries in Montana and South Dakota on June 3.
But Obama said this week the nomination fight may be over on May 20, if his expected win in Oregon that day gives him a majority of pledged delegates.
Despite the bitter campaign and rumors of animosity between the candidates, the rivals were cordial when they were both on the Senate floor this morning on Capitol Hill.
Obama walked over to Clinton to shake her hand and pat her on the arm. Later, Clinton traveled to Charleston, W.Va., where she toured a farmer's market and shook hands with voters.