Mayor Greg Nickels has an excuse for crossing that firefighters’ picket line in Providence: It’s across the street from the convention center where the annual Conference of Mayors is being held, and Nickels also entered the building through an enclosed second-floor walkway from his adjoining hotel, the Westin. Ergo, he didn’t “cross” the line. We didn’t say it was a good excuse. Just an excuse – one other mayors and the president of the United States weren’t about to make. Nickels has needed and gotten labor’s support in his past and current mayoral campaigns. But he also needs to become the new president of America’s mayors – a post he will officially accept this evening – since the title will look terrific on his campaign resume. One question: If Seattle voters this fall chose another leader, can you be president of the mayors when you’re not a mayor?