Denver, Colorado  - Just hours before Democratic delegates are to make a formal and symbolic choice between two historic firsts for their party - an African-American or woman presidential candidate - uncertainty prevailed Wednesday about just how the vote at the party convention would play out later Wednesday.

Senator Barack Obama, 47, has already secured the centre-left party's nomination after the long primary election season, but some supporters of his narrowly defeated rival Hillary Clinton are holding out against her emotional appeal Tuesday evening for them to swing their votes to Obama in the interest of party unity.

The practice in which delegates from each state vote to formally name the party's candidate has become largely ceremonial, and candidates that lost the primary elections have freed their delegates to support the prospective nominee. The move to even read Clinton's name during the vote at all aims to defuse tension from a historic series of state-by-state primaries pitting Clinton and Obama.

Over the past two days, Clinton's supporters have been passing around a petition on the convention floor in the hopes of getting 800 delegate signatures and using an obscure party rule that could force a full state-by-state roll call vote, John West, a Clinton volunteer from Illinois who is helping to organize the effort told Deutsche Presse-Agentur dpa.

If they get the needed number, they will give the petition to Clinton and let her decide what to do with it, West said. One and a half hours before the formal nominating session opens at 1900 GMT, Clinton is expected to have one final meeting with her delegates and urge them to vote for Obama.

"I want to make sure I get to vote! What is the purpose of this convention if it's not to vote," Maryland senator Mary Boergers told dpa.

All told, Clinton harvested more than 1,800 delegate votes during the brutal six-month-long primary season - the furthest any woman has come in securing a major party nomination.

The delegates represented 18 million votes who her supporters say will feel abandoned if Obama, Democratic National Committee chair Howard Dean and Speaker of the House of Representatives Nancy Pelosi have their way.

Obama's supporters are said to be pushing for a nomination by acclimation which would circumvent the embarassing prospect of each state casting so many votes for Clinton. Obama's spokeswoman Jenny Backus was quoted by Politico. com as saying they will support a state-by-state roll call, but Pelosi, as convention chair, could overrule that.

The vote is a traditional part of Democratic presidential conventions and allows each state to boast of its accomplishments and history and place its favourite candidate in the national limelight. It's important in mobilizing party loyalists in the November 4 general elections.

Allida Black, another Clinton delegate from Virginia who is ready to go to work for Obama after Hillary's inspirational speech Tuesday night, said she doesn't understand what Obama's campaign is afraid of.

"Hillary won't win all the delegates and this won't be a landslide," Black said, adding that many of Clinton's supporters will likely vote for Obama. "Afterwards we'll all go out and smash (presumptive Republican nominee John) McCain."

Boergers and Allida say their votes would not be against Obama but the fulfillment of an obligation they have to the voters who elected them as Clinton delegates.

Boergers also feels it's unfair that the Obama campaign has laid the responsibility to create party unity at the foot of Clinton and husband, former president Bill Clinton instead of directly reaching out to her supporters.

"I am so tired of people saying it's up to Hillary and Bill to bring people together," Boergers said, adding that they've already gone the extra mile.

Hillary's supporters already had to make use of another little- used party rule to force their candidate to submit her name for nomination. They collected 300 signatures on a petition requesting her name be put up on Wednesday's ballot. (dpa)