WASHINGTON (CNN) -- Ralph Nader is entering the presidential race as an independent, he announced Sunday, saying it is time for a "Jeffersonian revolution."

Ralph Nader is running for president as an independent.

"In the last few years, big money and the closing down of Washington against citizen groups prevent us from trying to improve our country. And I want everybody to have the right and opportunity to improve their country," he told reporters after an appearance announcing his candidacy on NBC's "Meet the Press."

Asked why he should be president, the longtime consumer advocate said, "Because I got things done." He cited a 40-year record, which he said includes saving "millions of lives," bringing about stricter protection for food and water and fighting corporate control over Washington.

Nader's decision, which did not come as a surprise to political watchers, marks his fourth straight White House bid -- fifth if his 1992 write-in campaign is included.

The two contenders for the Democratic nomination were quick to pounce.

"He thought that there was no difference between Al Gore and George Bush and, eight years later, I think people realize that Ralph did not know what he was talking about," Sen. Barack Obama said a town hall meeting Sunday. Watch Nader describe whom the Democrats should be "going after" »

Calling Nader's move "very unfortunate," Sen. Hillary Clinton told reporters, "I remember when he ran before. It didn't turn out very well for anybody -- especially our country."

"This time I hope it doesn't hurt anyone. I can't think of anybody that would vote for Sen. McCain who would vote for Ralph Nader," she said.

Nader was criticized by some Democrats in 2000 for allegedly pulling away support from Democrat Al Gore and helping George Bush win the White House.

Noting that he ran on the Green Party ticket that year, Clinton said Nader "prevented Al Gore from being the 'greenest' president we could have had."

Nader has long rejected his portrayal as a spoiler in the presidential race. In his NBC interview Sunday, he cited the Republican Party's economic policies, the Iraq war, and other issues, saying, "If the Democrats can't landslide the Republicans this year, they ought to just wrap up, close down, emerge in a different form."

But Clinton said, "Obviously, it is not helpful to whoever our Democratic nominee is. But, you know, it is a free country."

Nader said political consultants "have really messed up Hillary Clinton's campaign."

Long-shot GOP contender Mike Huckabee said Nader's entry would probably help his party.

"I think it always would probably pull votes away from the Democrats and not the Republicans, so naturally, Republicans would welcome his entry into the race," Huckabee said Sunday on CNN.

Nader said Thomas Jefferson believed that "when you lose your government, you've got to go into the electoral arena."

"A Jeffersonian revolution is needed in this country," he said.

Nader told NBC that great changes in U.S. history have come "through little parties that never won any national election."

"Dissent is the mother of ascent," he said. "And in that context I've decided to run for president."

Nader, who turns 74 this week, complained about the "paralysis of the government," which he said is under the control of corporate executives and lobbyists.

Obama also criticized Nader earlier this weekend. "My sense is that Mr. Nader is somebody who, if you don't listen and adopt all of his policies, thinks you're not substantive," he told reporters when asked about Nader's possible candidacy.

"He seems to have a pretty high opinion of his own work."

Obama said Nader "is a singular figure in American politics and has done as much as just about anyone for consumers."

"I don't mean to diminish that," he said. "There's a sense now that if someone's not hewing to the Ralph Nader agenda, he says they're lacking in some way."

Responding to those remarks, Nader called Obama "a person of substance" and "the first liberal evangelist in a long time" who "has run a good tactical campaign." But he accused Obama of censoring "his better instincts" on divisive issues.

Nader encouraged people to look at his campaign Web site, votenader.org, which he said discusses issues important to Americans that Obama and Sen. John McCain "are not addressing." E-mail to a friend

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