His victory had been widely assumed for weeks. But Clinton's declaration of interest in becoming his ticketmate was wholly unexpected.

She expressed it in a conference call with her state's congressional delegation after Nydia Velazquez, predicted Obama would have great difficulty winning the support of Hispanics and other voting blocs unless the former first lady was on the ticket.

"I am open to it" if it would help the party's prospects in November, Clinton replied, according to a participant who spoke on condition of anonymity because the call was private.

Obama's campaign had no reaction to Clinton's comments, which raised anew the prospect of what many Democrats have called a "Dream Ticket" that would put a black man and a woman on the same ballot.

The fast-paced developments unfolded as the long Democratic nominating struggle ended with primaries in Montana and South Dakota.

Only 31 delegates were at stake, the final few among the thousands that once drew Obama, Clinton and six other Democratic candidates into the campaign to replace President Bush and become the 44th US president.

Obama arranged an evening appearance in St. Paul, Minn., sending McCain an unmistakable message by claiming his victory in the very hall where the Arizonan will accept his party's nomination in early September.