Presidential rivals John Edwards, Dick Gephardt and John Kerry have found a common foe to unite them: Howard Dean (search). In a rare alliance, the three campaigns are working to deny the fellow Democratic candidate an endorsement from the Service Employees International Union (search), the largest in the AFL-CIO (search) with 1.6 million members.

The SEIU announced last week that its 63-member board would decide Thursday whether to back Dean, a former Vermont governor, or table the endorsement altogether.

"It's Dean or no one," said SEIU spokeswoman Sara Howard.

That announcement prompted top aides to Edwards, Gephardt and Kerry to convene a conference call during which they discussed whether Dean's endorsement could be blocked.

Comparing intelligence from their sources inside the union, the Edwards, Gephardt and Kerry campaigns determined that they still had an outside chance to stop Dean, according to senior officials with the three campaigns.

The officials, who spoke on condition of anonymity, differed on minor details such as how long last week's call lasted and what day it was placed. One official said it lasted 12 minutes; another said it was much longer.

They confirmed, however, that the strategists agreed that each campaign should call anybody who might help stop the endorsement. They denied any effort to coordinate the calls or messages, noting that each of the three candidates has a different constituency and appeal inside the union.

SEIU board members have been besieged by calls from the three campaigns, according to union officials.

The front-running Dean campaign has accused its rivals of mounting coordinated attacks before, saying the latest example was full-court criticism over remarks the former Vermont governor made about the Confederate flag (search).

The criticism leveled at Dean is no greater, however, than what past front-runners have experienced. Also, the notion of rival campaigns working in concert out of convenience is not unprecedented in American politics.

"No matter how much people might disdain their tactics of working together on this, it does point out how important this endorsement will be," said Dean campaign manager Joe Trippi.

The endorsement of SEIU means a potentially huge cadre of campaigners working on a candidate's behalf and possibly other union endorsements. Should Dean claim the prize next week, it would help him diversify his campaign, which largely has been Internet-driven and has attracted mostly younger, white middle- and upper-class voters.

SEIU is among the most racially and ethnically diverse labor unions, representing janitors, health care workers and other service employees. With health care a priority of the SEIU political machine, an endorsement also would help shield Dean from criticism that he has not always supported Medicare.