DES MOINES (AP) - A firefighters union is canceling a presidential candidate forum planned for Tuesday in Iowa City, because of conflicts with a congressional vote on a labor measure.

The International Association of Firefighters says it will reschedule the forum in September -- but after Labor Day. That way candidates won't miss voting on the bill, which would allow changes in the way workers vote on union membership.

Senators Hillary Rodham Clinton of New York, Chris Dodd of Connecticut and Barack Obama of Illinois were slated to speak to the group tomorrow. John Edwards, a former North Carolina senator, was set to speak Wednesday.

The firefighters union is a politically active group, and its endorsement helped boost John Kerry to the Democratic nomination in 2004. All the Democratic contenders actively court the group.