The Service Employees International Union has postponed its presidential endorsement until next month, underscoring divisions within the powerful labor group over front-runner Hillary Rodham Clinton, sentimental favorite John Edwards and latest star Barack Obama. Each of the top-tier candidates has support within the 1.8-million member union that includes janitors, hotel workers and truck drivers. SEIU backing is one of the most important labor endorsements available, with the organization donating more than $25 million, mostly to Democratic candidates, since 1989. Edwards has won over many of the union's leaders, yet that hasn't translated into an endorsement. The SEIU's executive board has decided to wait until October before deciding which candidate to endorse.