The Green Party presidential hopeful picked up the endorsement of a big nurses' union based in California, a state where his candidacy might have a significant impact on the general election.
The California Nurses Association, a 31,000-member union based in Oakland, praised Nader as committed to universal health care, patients' rights and expanded federal health insurance.
"He is the real thing, and he deserves the support of caregivers," Kay McVay, president of the association, said in a statement Wednesday.
Accepting the endorsement in Washington, Nader said traditional Democrats should welcome the prospect of the Green campaign helping to defeat Al Gore in November.
"If I was a dyed-in-the-wool Democrat, I'd be happy because that would give the Democratic Party a four-year cold shower for reconsidering their historical traditions at its highest levels and why it has abandoned these traditions," Nader said.
Some Democrats fear in a close race between Gore and Republican George W. Bush, Nader could tip the balance in the direction of the Texas governor.
Nader has been doing better than expected in some national polls, with some showing him at 4 to 5 percent.
Nader is expected to formally win the Green Party endorsement at the party's convention in Denver next week. He ran for the White House in 1996 but made little effort to campaign or raise funds and won few votes.
Earlier this month, Nader, for the second time, selected American Indian activist Winona LaDuke as his vice presidential running mate.
CBS Worldwide Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press and Reuters Limited contributed to this report