Joe Biden, a US Senate foreign-policy heavyweight, has joined the crowded field of Democrats competing for the party's 2008 nomination for president.
Senator Biden backs a partition plan for Iraq
Mr Biden, 64, joked about how many people are running for the White House as he announced his intentions.
"I'm the 800th candidate," he said on ABC television's Good Morning America.
He admitted he was not as prominent a figure as presumed party front-runners Hillary Clinton and Barack Obama, but touted his "ideas and experience".
Mr Biden said he was leaping directly into the race for president without first forming the customary exploratory committee.
"I'm not exploring. I'm in," he said on Wednesday morning.
He took over the chairmanship of the Senate foreign relations committee after the Democrats gained control of Congress in November's mid-term elections.
Senator Clinton is the presumed Democratic front-runner
He has been one of Washington's most prominent backers of partitioning Iraq into enclaves based on religion and ethnicity as a way of halting the violence there.
He touts the idea of a "federalised Iraq" as a "third way" between unilateral withdrawal of American troops and "staying the course".
He was a co-sponsor of a Senate resolution declaring that President George W Bush's plan to send an extra 21,500 troops to Iraq is "not in the national interest".
The foreign relations committee has approved the resolution opposing Mr Bush's move, and it is now awaiting a vote by the full Senate.
Mr Biden has been a senator representing the small north-eastern state of Delaware since the age of 30.
He ran for president briefly in 1988 but withdrew after he admitted to plagiarising speeches by Neil Kinnock, who was the leader of Britain's Labour party at the time.
He has a reputation as a forceful speaker, though he is often parodied in Washington for his long-winded questioning of witnesses in Senate hearings.
He is scheduled to appear soon on Comedy Central's The Daily Show.
Early polls show him trailing far behind Senators Clinton and Obama and former Senator John Edwards in the race for the Democratic nomination.