"She's just another bad version of (former President) Bill Clinton,'' Nader told KGO radio host Ronn Owens in San Francisco while on a book tour, as reported by the San Francisco Chronicle.
Nader has recently said he'll consider making a bid for the White House later this year.
"We're going to see what the Democrats come up with," he said.
Nader said Clinton, a Democratic front-runner for the White House, has "no political fortitude."
"Flatters, panders, coasting, front-runner, looking for a coronation, not taking on the huge waste in the military budget as a member of the armed services commission, never going after the corporate crimes against pensions, against workers. ... She has no political fortitude.''
The latest FOX News poll shows that 76 percent voters said they would never vote for Nader, while 44 percent said they would never cast a vote for Clinton.
Nader made an unsuccessful bid for president as the Green Party candidate in 2000, gaining more than 2.7 percent of the national popular vote. He gained less than one-half of 1 percent during a second run in 2004.
On another Democratic front-runner, Sen. Barack Obama of Illinois, Nader said he hasn't done much in the two years he has been in the Senate.
Obama "has capacity ... whether he gets specific in his proposals, that's what a lot of people are waiting for,'' Nader said. "He hasn't shown that much political fortitude ... in his two years in the Senate.''