John McCain effectively clinched the Republican nomination after primaries in four states, including Ohio and Texas, on 4 March.
What are the next steps?
Mr Obama's position as the presumptive Democratic nominee will not be confirmed until his rival, Hillary Clinton, concedes defeat.
"In the coming days, I'll be consulting with supporters and party leaders to determine how to move forward with the best interests of our party and our country guiding my way," she said after the final primaries on 3 June.
After this, Mr Obama called Mrs Clinton and suggested they "sit down when it makes sense to you". This meeting is not expected to happen immediately.
Does Mrs Clinton want something from Mr Obama?
She has indicated she would be open to the idea of being Mr Obama's running mate.
She could also be angling for another job in Mr Obama's administration, if he is elected.
It's also reported that she would like Mr Obama to help her pay back some of her campaign's debts - and that Bill Clinton could be keen on some kind of role in the Democratic general election campaign.
Mrs Clinton claims to have won a majority of the popular vote. The figures are are open to dispute, however.
According to the RealClearPolitics website, Mrs Clinton has narrowly won the popular vote if you (a) include the figures for Michigan (ascribing all votes for "uncommitted" to Barack Obama) and (b) ignore the votes cast in Iowa, Nevada, Washington and Maine, which have not released figures.
Including estimates for the votes cast in these states, RealClearPolitics' says Mr Obama has narrowly won the popular vote.
When will the remaining super-delegates make up their mind?
About 150 super-delegates have yet to decide which candidate to back.
They are not obliged to do so until the party's national convention in August, where the nominee will be chosen. (And they can change their mind as often as they like).
Did the ruling on Florida and Michigan delegates alter the situation?
Democratic Party officials decided on 31 May to allow previously barred delegates from Florida and Michigan to attend the convention, but with just half of their voting power.
This increased the number of delegates needed to clinch the nomination from 2,026 to 2,118.
It gave Mrs Clinton 69 delegates in Michigan, compared to Mr Obama's 59. And in Florida, she gained 105 delegates to Mr Obama's 67.
This reduced Mr Obama's lead by 48, but the delegates from Michigan and Florida will only have half a vote each, so Mrs Clinton's real gain was just 24 delegate votes.
She has the right to appeal against the decision.
What happens now that the Democrats appear to have a presumptive nominee?
The focus will switch to the general election battle between John McCain and his Democratic rival.
Mr Obama and Mr McCain have already begun sparring over Iraq and national security issues.
They have also been focusing attention on the states that could vote either way in November. The list includes large states such as Florida, Michigan, Ohio and Pennsylvania, and smaller ones such as Colorado, Iowa, Minnesota, Nevada, New Mexico, New Hampshire, Oregon, Virginia and Wisconsin.
What is the advantage for Mr McCain of having secured the Republican nomination already?
It gives him a big head start. He has had two months to plan his general election campaign, and raise funds, while the Democrats continued fighting each other for the nomination.
On the other hand, the close Democratic nomination race has caused lots of people to register as Democratic voters, which could give the party a big advantage in November.
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