A group of American firefighters opposed to Rudy Giuliani, the Republican front-runner in the 2008 presidential race, are planning to run damaging adverts expressing scepticism about his 9/11 leadership.

The deputy New York fire chief, Jim Riches, whose son, also a firefighter, was killed in the Twin Towers' collapse, said: "TV made him a hero, and we'll use TV to take him down."

Riches and other New York firefighters have been voicing opposition to Giuliani, mayor of the city on 9/11, since he launched his campaign for the Republican nomination earlier this year. But they are now seeking to set up an official organisation that will fund television adverts in a campaign similar to the one that undermined John Kerry in 2004, the Swift Boat Veterans for Truth, which challenged his Vietnam record.

The organisation would also include the families of 9/11 victims.

Giuliani is running his campaign on the back of the largely favourable media he received at the time, with his leadership in New York contrasting with President George Bush's hesitancy. But the firefighters and victims' families complain that Giuliani failed to heed the warning of an attack on the World Trade Centre almost a decade earlier.

The firefighters also say the crews had poor radios and, as a result, some did not hear calls to evacuate the site.

Families accuse Giuliani of rushing the clear-up of Ground Zero, denying them the chance to exhaust the search for bodies. There is unease, too, that Giuliani is using 9/11 as a political platform.

Riches told ABC News: "We don't want him running on 9/11 or the bodies of all these dead people or my dead son saying that he did a great job that day."

The group is planning trips to Iowa and other key states voting in caucuses and primaries in January to campaign against Giuliani. The former mayor, who has a clear poll lead over his Republican rivals nationally, had initially decided to concentrate on big states such as California. But fearful of his main rivals gaining unstoppable momentum by wins in Iowa and other key places, Giuliani's team has been building up a network in the state.