WASHINGTON Barack Obama was on the brink of winning the Democratic presidential nomination on Tuesday, claiming the support of a steady parade of uncommitted delegates and pushing rival Hillary Clinton to the verge of defeat.
At least 20 superdelegates -- party officials free to back any candidate at the August nominating convention -- and 10 delegates pledged to former rival John Edwards announced their support of Obama, who would be the first black presidential nominee of a major U.S. political party.
That put Obama fewer than 15 delegates from the 2,118 he needs to face Republican John McCain in November. He could clinch the honor after polls close in the final two contests in Montana and South Dakota, which have 31 delegates at stake.
Facing defeat, Clinton told a conference call with New York members of Congress that she would be open to becoming Obama's vice presidential running mate.
But her campaign said she did not plan to concede to Obama at a rally in New York later on Tuesday.
"The nomination fight goes on until somebody gets the magic number and that isn't there today and that is not at all what Senator Clinton is going to talk about tonight," Clinton campaign chairman Terry McAuliffe told CNN.
Voting ends in South Dakota at 7 p.m. MDT/9 p.m. EDT (0100 GMT), and in Montana an hour later, with results expected shortly afterward.
Obama planned a victory celebration to kick off the general election campaign against McCain after the South Dakota and Montana polls close. He will hold it at the St. Paul, Minnesota, hockey arena that will host the Republican convention in September.
His campaign had urged the approximately 150 undecided superdelegates to make their endorsement before the voting ends, so the delegates he wins in the two states could allow him to clinch the Democratic race.
A steady flow of superdelegates made their announcements throughout the day. Former President Jimmy Carter will endorse Obama when the polls close, the Carter Center said.
Rep. Maxine Waters of California switched from Clinton to Obama, saying "now is the time for us to unite so that real change is possible in November."
U.S. Rep. James Clyburn of South Carolina, third-ranking Democrat in the U.S. House of Representatives and the top-ranking black member of Congress, formally announced his support for Obama and urged other superdelegates to announce their decisions so Obama could wrap things up by day's end.
'PROCESS DRAWS TO A CLOSE'
"Today the primary process draws to a close," Clyburn said. "I believe the time has come for all unpledged delegates to make their choices known."
A group of 17 uncommitted Senate Democrats met to discuss the timing of a potential endorsement of Obama. They will meet again on Wednesday but are not expected to make their announcements by Tuesday night, a Senate aide said.
"Senator Clinton needs to be left alone. Let's get through the primary process and let this week work it's course," Senate Democratic Leader Harry Reid told reporters.
Clinton and her campaign have sent mixed signals over the last two days about how long she would stay in a presidential race that she began as a heavy favorite but now has little chance of winning.
During the conference call on Tuesday, she was asked about running as the No. 2 to Obama. "I am open to it," she replied, according to a party aide.
Her campaign backed away, saying she was asked if she was open to the idea of being vice president and "repeated what she has said before: she would do whatever she could to ensure that Democrats take the White House back and defeat John McCain."
Campaigning in South Dakota on Monday, Clinton said the end of the voting marked "the beginning of a new phase of the campaign" in which she will seek to convince superdelegates she would be the strongest candidate against McCain in November.
With no more campaign trips to plan, workers who handle Clinton's advance travel arrangements have been told to go to New York or head home until further notice, aides said.
Both Obama and Clinton will speak to a pro-Israel lobbying group in Washington on Wednesday, and Obama said he expected to be talking to Clinton again soon.
He said he told her in a phone conversation on Sunday that "once the dust settled I was looking forward to meeting with her at a time and place of her choosing."
(Additional reporting by Caren Bohan, Doina Chiacu, Thomas Ferraro, Jackie Frank and Ellen Wulfhorst; Editing by David Wiessler)
(To read more about the U.S. political campaign, visit Reuters "Tales from the Trail: 2008" online at blogs.reuters.com/trail08/)