US consumer champion Ralph Nader has announced he will run for president in the November 2004 election.
Some Democrats believe Al Gore would be president, but for Mr Nader
"I've decided to run as an independent candidate," Mr Nader told NBC television's Meet the Press programme.
He stood as a Green Party candidate in the 2000 election and many Democrats blame him for handing the election to George W Bush.
An anti-establishment figure, Mr Nader won less than 3% of the vote in the last US presidential election.
But many Democrats believe he took just enough support from Al Gore to hand the contest to Mr Bush, especially in the crucial state of Florida, where Mr Nader won 97,488 votes.
Mr Bush beat Al Gore by just 537 votes in Florida.
Announcing his decision, Mr Nader said: "This country has a lot of problems and injustices which it doesn't deserve.
"There is too much power and wealth in too little hands."
Mr Nader said he made his decision "after careful thought".
"Both parties are flunking, Republicans with a D minus and Democrats with a D plus," he said.
"It's time to change the equation."
There has been an angry backlash against Mr Nader, who turns 70 this month, since the controversial results in the 2000 election.
Thousands of people have cancelled donations to consumer groups Mr Nader founded or supported.
A website has been set up called Ralph Don't Run.
And with this election forecast to be every bit as close as the last, friends and leading Democrats pleaded with him to stay out of the race, says the BBC's Jon Leyne in Washington.
"It's an act of total ego," said New Mexico Governor Bill Richardson, a Democrat and former member of President Bill Clinton's Cabinet.
Ed Gillespie, chairman of the Republican National
Committee, said: "The fact is that if Ralph Nader runs, President Bush is going to be re-elected, and if Ralph Nader doesn't run, President Bush is going to be
Mr Nader said those arguing against him running were contemptuous about democracy.
"This is a fight for all third-party candidates," he said. "This is not a democracy that can be controlled by two parties in the grip of corporate interest."
But our correspondent says that with so much antagonism towards Mr Nader, even among his former supporters, the independent candidate's first problem will be getting enough signatures even to get himself onto the ballot in many states.
Super Tuesday visits
The two leading Democrats hoping to challenge Mr Bush in the November election campaigned on Sunday in states conducting primaries on 2 March - so-called Super Tuesday, when 10 states are holding contests.
Both men visited black community churches. John Edwards, the senator for North Carolina, visited a church in Columbus, Ohio, while the frontrunner, Massachusetts Senator John Kerry, visited a church in Atlanta, Georgia, where Martin Luther King Jr once preached.
He also held a town hall meeting in the city, where he criticised Mr Bush over what he called the biggest transfer of wealth from the middle class to the wealthy in the country's history.
Both men also made separate appearances on ABC television's This Week programme.