WASHINGTON : Democrat Barack Obama looked set Tuesday to pass an electoral milestone in his historic White House quest but Hillary Clinton's camp warned him not to declare "mission accomplished."

The two rivals for the Democratic nomination were facing primary voters in Kentucky and Oregon on the home stretch of their dramatic race, with Obama looking to clinch a symbolic majority of elected delegates.

Latest polls suggested the former first lady would win in Kentucky and front-runner Obama, 46, was tipped to win in Oregon. But the two states' combined total of 103 delegates was unlikely to alter the epic race's end-game.

Obama's campaign billed Tuesday as a watershed moment on his apparent march towards the nomination, saying he would likely emerge from the twin primaries with a majority of delegates elected in all valid Democratic contests.

But Clinton, 60, warned the first-term Illinois senator against premature victory celebrations, even though he leads in total wins, pledged delegates, "superdelegates" and the popular vote in certified primaries and caucuses.

"This is nowhere near over," New York Senator Clinton said, campaigning in Kentucky Monday, defying calls for her to quit the race and allow Obama to focus on an evolving general election showdown with Republican John McCain.

Most experts believe only a monumental error by Obama can reverse the daily flow of superdelegates or top party officials towards his cause, and stop him becoming the first African-American presidential nominee in history.

First polls were to open at 6:00am (1000 GMT) and stay open for 12 hours in Kentucky, which spans two time zones, and is an east-central state known for coal mining, thoroughbred race horses and pastures of bluegrass.

Opinion surveys suggest Clinton's coalition of white, blue collar, female and older voters will hold firm and carry her to victory.

Obama is however favoured in Oregon, a sparsely populated, liberal state on the western seaboard where voting was to end at 8:00pm Pacific time (0300 GMT).

In a sign he is looking already to November, Obama was to spend Tuesday evening in Iowa, where he catapulted into contention with a shock win in the year's first nominating showdown in January.

The homecoming of sorts will offer an evocative pivot for Obama to suggest victory is assured in the Democratic race, and to tout for the midwestern swing state's support in the general election.

Obama's campaign manager David Plouffe said his boss would likely secure a majority of pledged delegates once Kentucky and Oregon results were in.

"A clear majority of elected delegates will send an unmistakable message -- the people have spoken, and they are ready for change," he wrote in a fundraising email.

"As we near victory in one contest, the next challenge is already heating up," Plouffe said.

Obama's campaign says he needs only 16 more pledged delegates to reach that majority, which does not count the nearly 800 superdelegates free to vote for the nominee of their choice.

But Clinton's campaign said such talk is a "slap in the face" for her millions of supporters and an insult to voters in Puerto Rico, Montana and South Dakota, which have yet to weigh in.

Obama added endorsements from another five superdelegates Monday, and is now less than 120 total delegates away from the total of 2,025 needed to secure the nomination, according to independent website RealClearPolitics.

But the Clinton campaign maintains the real finish line is 2,209 -- including Michigan and Florida, which held primaries but had delegates stripped away by Democratic bosses after breaking scheduling rules.

In a symbolic moment, Robert Byrd of West Virginia, a 90-year-old Democratic titan who has spent half a century in the Senate, backed Obama, despite his beloved home state voting overwhelmingly for Clinton last week.

Byrd was briefly a member of the white supremacist Ku Klux Klan but long ago renounced his early racist leanings, and his support of Obama bolsters the Illinois senator's calls for reconciliation.

McCain, also eyeing a potential general election matchup, meanwhile sharpened a foreign policy assault on Obama, accusing him of recklessly minimizing the threat from Iran.

- AFP/ir