Barack Obama is hoping to take a step towards clinching the Democratic Party's US presidential nomination, as Kentucky and Oregon hold primary votes.
Barack Obama is hoping to take a step towards clinching the Democratic Party's US presidential nomination, as Kentucky and Oregon hold primary votes.
The Illinois senator is growing closer to securing enough support to end the lengthy contest with Hillary Clinton.
The Illinois senator is growing closer to securing enough support to end the lengthy contest with Hillary Clinton.
In the primaries so far, he has won more of the delegates who will choose the party's nominee at the Democratic National Convention this summer.
In the primaries so far, he has won more of the delegates who will choose the party's nominee at the Democratic National Convention this summer.
Opinion polls suggest Mr Obama will win in Oregon and Mrs Clinton in Kentucky.
Opinion polls suggest Mr Obama will win in Oregon and Mrs Clinton in Kentucky.
Senator Clinton insists the race is not yet over, but BBC correspondents say Mr Obama is keen to suggest the opposite.
Senator Clinton insists the race is not yet over, but BBC correspondents say Mr Obama is keen to suggest the opposite.
Mr Obama also leads in terms of the so-called "super-delegates", who get an automatic vote at the convention by virtue of holding a senior position in the party.
Mr Obama also leads in terms of the so-called "super-delegates", who get an automatic vote at the convention by virtue of holding a senior position in the party.
Tuesday's primary votes are unlikely to push Mr Obama over the 2,026 overall delegates needed to win the party's nomination.
Tuesday's primary votes are unlikely to push Mr Obama over the 2,026 overall delegates needed to win the party's nomination.
However, he is expected to secure a majority of the "pledged" delegates - those secured via primary votes - at stake.
However, he is expected to secure a majority of the "pledged" delegates - those secured via primary votes - at stake.
This milestone could encourage those super-delegates who remain undecided to back him.
This milestone could encourage those super-delegates who remain undecided to back him.
'Slap in the face'
'Slap in the face'
The first polls open in Kentucky at 0600 local time (1000 GMT) and will stay open for 12 hours.
The first polls open in Kentucky at 0600 local time (1000 GMT) and will stay open for 12 hours.
DEMOCRATIC DELEGATES Total delegates (pledged and super-delegates) needed for nomination: 2,026Total pledged delegates at stake: 3,253Total super-delegates at stake: 797Total delegate tally for Barack Obama: 1,915Pledged delegate tally for Mr Obama: 1,610.5Total delegate tally for Hillary Clinton: 1,721Pledged delegate tally for Mrs Clinton: 1,443 Source: Associated Press, as of 0900 BST on 20 May
DEMOCRATIC DELEGATES Total delegates (pledged and super-delegates) needed for nomination: 2,026Total pledged delegates at stake: 3,253Total super-delegates at stake: 797Total delegate tally for Barack Obama: 1,915Pledged delegate tally for Mr Obama: 1,610.5Total delegate tally for Hillary Clinton: 1,721Pledged delegate tally for Mrs Clinton: 1,443 Source: Associated Press, as of 0900 BST on 20 May
Voters in Oregon, which has a mail-in voting system, have until 2000 local time (0300 GMT) to return their ballots to election offices.
Voters in Oregon, which has a mail-in voting system, have until 2000 local time (0300 GMT) to return their ballots to election offices.
Mrs Clinton has focused her campaign efforts on Kentucky, where she held a big lead in opinion polls going into Tuesday's vote.
Mrs Clinton has focused her campaign efforts on Kentucky, where she held a big lead in opinion polls going into Tuesday's vote.
"This is nowhere near over," she told supporters at a rally on Monday.
"This is nowhere near over," she told supporters at a rally on Monday.
Her campaign warned Mr Obama against declaring premature victory, saying it would be a "slap in the face" to her millions of supporters so far and those states yet to vote.
Her campaign warned Mr Obama against declaring premature victory, saying it would be a "slap in the face" to her millions of supporters so far and those states yet to vote.
Mr Obama's aides are increasingly pushing the message that he has all but wrapped up the Democratic nomination and will be the one to face Republican John McCain in November's general election.
Mr Obama's aides are increasingly pushing the message that he has all but wrapped up the Democratic nomination and will be the one to face Republican John McCain in November's general election.
"As we near victory in one contest, the next challenge is already heating up," campaign manager David Plouffe wrote in an e-mail to supporters on Monday.
"As we near victory in one contest, the next challenge is already heating up," campaign manager David Plouffe wrote in an e-mail to supporters on Monday.
"President Bush and Senator McCain have begun co-ordinating their attacks on Barack Obama in an effort to extend their failed policies for a third term."
"President Bush and Senator McCain have begun co-ordinating their attacks on Barack Obama in an effort to extend their failed policies for a third term."
'Reckless judgment'
'Reckless judgment'
Mr Obama is due to give a speech in Iowa - scene of his first primary victory back in early January - before the results are even in from Tuesday's primaries.
Mr Obama is due to give a speech in Iowa - scene of his first primary victory back in early January - before the results are even in from Tuesday's primaries.
Hillary Clinton insists that the battle to run for president is not yet over
Hillary Clinton insists that the battle to run for president is not yet over
The obvious intention, BBC North America editor Justin Webb says, is to suggest without quite saying so that the primary season is closing and the general election race is beginning.
The obvious intention, BBC North America editor Justin Webb says, is to suggest without quite saying so that the primary season is closing and the general election race is beginning.
Our correspondent adds that although Mrs Clinton insists the race is still on, the truth is that the undecided super-delegates are moving steadily towards Mr Obama.
Our correspondent adds that although Mrs Clinton insists the race is still on, the truth is that the undecided super-delegates are moving steadily towards Mr Obama.
According to the Associated Press news agency, he picked up six more on Monday, taking his total including super-delegates to 1,915 going into Tuesday's polls, with 1,721 for Mrs Clinton.
According to the Associated Press news agency, he picked up six more on Monday, taking his total including super-delegates to 1,915 going into Tuesday's polls, with 1,721 for Mrs Clinton.
Once the 103 pledged delegates at stake in Kentucky and Oregon have been divided between the two rivals, Mr Obama may be only 50 to 75 short of the 2,026 delegates needed, AP says.
Once the 103 pledged delegates at stake in Kentucky and Oregon have been divided between the two rivals, Mr Obama may be only 50 to 75 short of the 2,026 delegates needed, AP says.
Mr McCain wrapped up the Republican Party's nomination in March
Mr McCain wrapped up the Republican Party's nomination in March
Mr McCain, senator for Arizona, has already begun targeting Mr Obama as his probable opponent in November.
Mr McCain, senator for Arizona, has already begun targeting Mr Obama as his probable opponent in November.
Monday saw him criticise Mr Obama over comments he made a day earlier suggesting that Iran does not pose the same level of threat to the US as the former Soviet Union once did.
Monday saw him criticise Mr Obama over comments he made a day earlier suggesting that Iran does not pose the same level of threat to the US as the former Soviet Union once did.
"Such a statement betrays the depth of Senator Obama's inexperience and reckless judgment," Mr McCain said.
"Such a statement betrays the depth of Senator Obama's inexperience and reckless judgment," Mr McCain said.
"These are very serious deficiencies for an American president to possess."
"These are very serious deficiencies for an American president to possess."
In response, Mr Obama said that while Iran was a "grave threat", it had no nuclear weapons whereas the Soviet Union "had thousands".
In response, Mr Obama said that while Iran was a "grave threat", it had no nuclear weapons whereas the Soviet Union "had thousands".
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