Denver, Colorado - Veteran Senator Joe Biden on Wednesday formally accepted the Democratic Party's offer to be vice president to Barack Obama, who made a surprise appearance at the party's convention a day before he plans to give his own acceptance speech.

"I want everybody to now understand why I am so proud to have Joe Biden ... and the whole Biden family with me on this journey to take America back," Obama said in brief remarks on stage at the Denver, Colorado convention.

It marked a rare appearance for a Democratic candidate before his own formal acceptance speech, which Obama plans to make Thursday before 75,000 people at Invesco Field stadium in Denver.

"We want to open up this convention to make sure that everybody who wants to come can join in the party," said Obama, who was formally nominated by the convention's 4,000 delegates Wednesday as the first African-American candidate for a major US political party.

Obama also praised both Hillary and Bill Clinton for their ringing endorsements of his candidacy in their own convention speeches, something which had until now been a large question mark given the party's long and bruising primary battle.

"If I'm not mistaken, Hillary Clinton rocked the house last night," Obama said. "President Bill Clinton reminded us of what it's like when you have a president who actually puts people first."

Former president Clinton earlier Wednesday threw his support fully behind Obama, saying the 47-year-old Illinois senator was "ready to lead America and restore American leadership in the world."

"He has shown a clear grasp of our foreign policy and national security challenges, and a firm commitment to rebuild our badly strained military," Clinton said. "He will choose diplomacy first and military force as a last resort."

Biden, in accepting the vice presidential nomination, also called on the country to back a "wise leader" who could restore the United States' economy and reputation.

The long-time Delaware senator spoke of his own working class roots and offered a moving description of the kinds of "kitchen table" discussions US families were having during the troubled economic times.

Obama's campaign hopes Biden, one of the party's leading voices on foreign policy, will not only boost the Democratic ticket's credentials on national security, but that he can also appeal to lower class voters skeptical of Obama, the first African-American candidate ever nominated by a major US political party.

Biden hammered presumptive Republican nominee John McCain for promising "more of the same" policies of President George W Bush. He criticized McCain, a Vietnam War hero, for being "wrong" on all of the important issues of national security, including the war in Iraq.

"The choice in this election is clear. These times require more than a good soldier - they require a wise leader. A leader who can deliver change," Biden said. "Barack Obama will deliver that change."

Republicans have a chance to push back on that message at their own party convention to nominate McCain next week. (dpa)