The U.S. Democratic Party's National Convention opens later Monday in Denver, Colorado, where later this week the first-ever African-American candidate will accept his party's nomination for president of the United States.

One-time U.S. democratic presidential hopeful Hillary Clinton spoke ahead of the convention Monday morning, calling on her supporters to unite behind Barack Obama.

Clinton urged her supporters to "work as hard for Barack Obama as you did for me."

Senator Clinton also denounced recent ads from the Republican candidate, John McCain's campaign, which portray the Democratic party as deeply divided.

Meanwhile, a recent survey of Clinton's supporters shows one third plan to vote for either John McCain or a third-party candidate - rather than for Senator Obama.

Resentment among many Clinton backers still remains coming into the convention, although the U.S. Senator and former first lady has said she will release her delegates in a gesture of unity.  

Clinton is to deliver a keynote address at the convention on Tuesday evening.  

Senator Obama's wife, Michelle, will deliver this evening's keynote address, formally introducing her husband to the nation.

Delegates will also pay tribute to ailing Democratic Senator Ted Kennedy, who is suffering from brain cancer.  He and his family are to attend the convention, though it is not yet clear if he will speak.

The convention consists of four days of rallies, speeches and meetings, capped by Obama's acceptance speech on Thursday.

The Republican Party holds its national convention next week.

Some information for this report was provided by AP.