Leading in the popular vote, the number of states won and pledged delegates, Obama had hoped to wrap up the nomination earlier in the spring to begin the general election campaign against Republican Sen. John McCain. But Clinton won the last major primaries in Ohio and Texas March 4 and has vowed to stay in until the party's convention this August in Denver.
Since neither candidate will finish with enough delegates to clinch the nomination, both candidates are wooing "superdelegates" - elected officials and party activists who may back any candidate they wish.
Obama is counting on superdelegates to back the winner of the popular vote and pledged delegate count, a notion even some Clinton supporters agree with. He also figures prominent Democrats would be loath to deny the nomination to a black man who leads in delegates and votes. Clinton is hoping to close the gap with Obama in the final contests and persuade superdelegates that she would be the stronger general election candidate against McCain.
Here's a look at the final 10 contests: --- -PENNSYLVANIA (Tuesday, 158 pledged delegates) The largest of the remaining states, Pennsylvania appears tailor-made for Clinton even as polls show her once commanding lead narrowing to single digits. Most analysts predict a win for the former first lady; a loss would effectively doom her candidacy. Even a narrow Clinton victory would be viewed as a tactical triumph for Obama.
While Pennsylvania's economy may not be as fragile as that of neighboring Ohio, which Clinton won by 9 percentage points, the state has seen a similar erosion of its manufacturing base and a loss of thousands of blue-collar jobs.
Pennsylvania also has a large white, working-class population and other groups that Clinton has attracted. It has the third oldest population of any state and is home to a significant number of ethnic Catholic voters. The state's politically savvy governor, Ed Rendell, backs Clinton.
Obama is competing in Pennsylvania and was on a five-day bus and train tour of the state. He has 30 field offices to Clinton's 24 and has outspent her more than 2-to-1 on television advertising.
The New York senator is hoping Obama's comments about bitter voters in small town will undercut his support.
Geographically, Clinton is strongest in western Pennsylvania including the Pittsburgh area, Johnstown and Erie, and in northeastern communities like Scranton, Wilkes-Barre, and Carbondale. Obama's base is in Philadelphia, with its large population of blacks, affluent voters and students. He also performs well in Harrisburg, the state capital.
Both campaigns view the northern and southern Philadelphia suburbs and the Lehigh Valley as key battleground areas.
Only registered Democrats can vote in the primary.
--- -GUAM (May 3, four delegates) Neither campaign has expended loads of effort on the tiny Pacific island, but the Obama campaign has deployed three staff members there and the Clinton camp has been organizing under the leadership of a popular Guam state senator, Tina Muna Barnes.
--- -NORTH CAROLINA (May 6, 115 delegates) With its large population of black voters and well-populated liberal enclaves, Obama is heavily favored to win North Carolina. Recent polling shows him with a double-digit lead over Clinton, and both candidates vying for the state's Democratic gubernatorial nomination have endorsed the Illinois senator.
Former North Carolina Sen. John Edwards, who also sought the nomination, has declined both candidates' endorsement requests. He's kept a low profile since leaving the race, and it's unlikely he will appear with either candidate leading up to the primary.
Obama has been advertising on television in North Carolina since late March. Clinton went up recently with an unconventional one-minute ad in which she invited North Carolinians to submit questions that she would answer in subsequent ads leading up to the primary. More than 10,000 questions have been submitted so far.
Obama is targeting the state's five major urban areas - Charlotte, Durham, Greensboro, Winston-Salem and Raleigh, the state capital. Clinton is eyeing the more rural regions of the state and military communities such as Fayetteville.
Unaffiliated voters can vote in either party's contest. The state has heavy early voting that began Thursday.
--- -INDIANA (May 6, 72 delegates) This is a true Democratic battleground state, with a demographic mix that plays to each candidate's strengths. Recent polling shows a close race.
Indiana shares a border with Obama's home state of Illinois, and he has won every other border state so far. About 20 percent of the state is in the Chicago media market, another plus for Obama. He is strongest in northern Indiana, with its urban pockets and large black population. He also does well in Indianapolis and in Bloomington, home of Indiana University. Clinton is popular farther south, especially among rural voters and in white working-class communities like those along the Ohio River.
Both candidates are airing TV ads and have made repeated trips to the state. Clinton has the support of Evan Bayh, the state's popular senator and former governor.
Indiana has an open primary.
--- -WEST VIRGINIA (May 13, 28 delegates) Both campaigns view the state as extremely favorable to Clinton - it has no urban core, relatively few black voters, and a large white working-class population. Public polling shows Clinton with a double-digit lead over Obama.
Obama has the endorsement of Sen. Jay Rockefeller and the campaign plans to deploy staff soon. Both candidates are competing for the endorsement of the United Mine Workers, who are influential in the state.
Unaffiliated and independent voters can vote in the primary.
--- -OREGON (May 20, 52 delegates) Obama is favored to win in a state with much of the population concentrated in Portland, liberal coastal areas and the university community of Eugene. Still, the Clinton campaign is holding out hope for a strong showing among rural and working-class voters in cities such as Medford and communities in central and eastern Oregon. Polling shows Obama up by double digits; Clinton hopes to trim that lead.
The state's primary is conducted almost entirely by mail, beginning the first week in May when voters might be influenced by the outcome of other primaries.
--- -KENTUCKY (May 20, 51 delegates) Like neighboring West Virginia, Kentucky is demographically favorable for Clinton although the state's major cities, Louisville and Lexington, are home to a number of black voters.
Only registered Democrats can vote in the primary. There is no early voting in Kentucky.
--- -PUERTO RICO (June 1, 55 delegates) Clinton has run much stronger among Hispanics throughout the primary, but Obama is contesting the island commonwealth.
Puerto Ricans cannot vote in the U.S. general election, but a high turnout is expected in the primary.
Much of the local political culture revolves around the island's relationship to the U.S., with three subgroups - one that wants Puerto Rico to become a U.S. state, one that wants it to become an independent country and one that wants it to remain a commonwealth. Both Clinton and Obama want to see Puerto Rico's status resolved based on the preference of a majority of its citizens.
Obama suffered an embarrassment recently when his most prominent supporter in Puerto Rico, Gov. Anibal Acevedo Vila, was charged with conspiring to violate federal campaign laws, defraud the Internal Revenue Service, and give false testimony to the FBI.
--- -MONTANA (June 3, 16 delegates) Both Obama and Clinton have visited Montana and the campaigns have deployed staff here. Both campaigns believe the state is a battleground, perhaps somewhat more favorable to Obama since he has shown significant strength in caucuses across other Great Plains states. But Clinton has done well in primaries, giving the campaign hope that she can perform well here.
The state holds an open primary.
--- -SOUTH DAKOTA (June 3, 15 delegates) Obama has significant advantages in South Dakota, starting with a string of major endorsements. Former Senate Majority Leader Tom Daschle backs the Illinois senator, as does current Sen. Tim Johnson and Rep. Stephanie Herseth-Sandlin, the state's lone member of Congress.
Both campaigns are actively courting native American tribal leaders and the candidates have promised to make native American issues a priority if elected president. The state holds an open primary.
