The independent presidential candidate sent an open letter to Moore on Friday pleading with him to support Nader's third-party candidacy, as Moore did in 2000.
"Ok Michael, you've had your realpolitik (search) fling with ex-General Wesley Clark," Nader writes. "Your endorsed presidential candidate in the Democratic primaries has withdrawn. It is time for you to come home, to join your buddies and resume your only genuine role which is that of defiance and resistance."
Nader spokesman Kevin Zeese said Moore has seen the letter but has not responded. Moore could not immediately be reached for comment Friday.
Dozens of politically active celebrities, including Moore and actors Tim Robbins (search) and Susan Sarandon (search), campaigned with Nader four years ago. But many former supporters who believe their vote for Nader helped put President Bush in the White House have shunned the consumer activist this time around.
Moore, who won an Oscar for his 2002 documentary "Bowling for Columbine," actively campaigned for Clark until the retired general dropped out of the race in February. Moore has not endorsed another presidential candidate.
Zeese said the letter is Nader "just having a little fun" and should not be viewed as a sign the campaign is having trouble garnering support.
"This is definitely not a desperate call," Zeese said. "Michael is symbolic of the peace and justice movement. Is the peace and justice movement going to put its tail between its legs and support a candidate who doesn't support their views?"