There is a line between pushing a cause and indulging a grudge, and Providence firefighters have jumped over it with the kind of gleeful indifference often seen in hopscotch.

This is petty, childish, stupid and counterproductive. The firefighters are looking a little out of touch.

When union strategy becomes nothing more than “embarrass the mayor at all costs” there is a very good chance that the collateral damage will spill all over the place.

And it has. What the firefighters union has done by promising to picket this week’s gathering of the U.S. Conference of Mayors is not hurt Mayor David Cicilline or advance its labor demands. It has embarrassed the city its members are supposed to serve. It has embarrassed the state. It has embarrassed itself.

The union has denied mayors from all over the country the opportunity to exchange ideas with federal officials. It has shut down a small but vital exercise in open government.

It has done damage that will last far beyond the weekend. Opportunities will be lost.

Sadly, the federal government has aided and abetted as more than 100 federal officials and staff members have said they will not attend because of the threatened action by the firefighters.

Vice President Joe Biden, Attorney General Eric Holder and Cabinet members were among those scheduled to be in Providence this week. And now they will not be here because of what seems an almost automatic bow to the union and a rush to keep up pro-labor credentials. There seems to have been no attempt by federal officials to understand the situation in Providence, to learn that there is far more at work here than an aggrieved union making its case in the streets. It’s personal and it brings no credit to the cause.

The Conference of Mayors meeting is the kind of event that draws demonstrators. It is a chance some can’t pass up to get their message in front of lots of important people and lots of cameras. The firefighters could have been part of the mix. Instead, they will be a sad sideshow on the sidewalk, accomplishing little beyond a group howl.

I’m a union guy, but when a union stifles the free and open exchange of ideas, when it lets its legitimate issues become overshadowed by personal vendetta, then it calls into question its role as a legitimate voice of its members.

This could have been a very good week for Providence and for Rhode Island. The conference is a huge event and it means people from all over the country will be getting their first look at this place. Unfortunately, that first look will include something really dumb that will tarnish the people who created it for a long, long time.

Just about everybody fouled up here except, perhaps, the man at the center of it all. Cicilline worked hard to bring the conference to Providence. It is a big boost for the city, even in its vastly diminished form. And he refused to give in to the demands of the firefighters union that he not attend as a condition for the union not demonstrating. He really couldn’t give in and remain an effective mayor.

Cicilline also went public with the latest contract offer to the firefighters — a very smart move in hard economic times. It is, in effect, a call for a reality check.

The firefighters really get nothing from this, except a rock-solid claim on messing up a wonderful opportunity for their city and state.

They and the good work they do every day of the year are diminished by it.

bkerr@projo.com