The first face-to-face confrontation of the 2008 presidential race is looming over a US Senate inquiry into health problems suffered by workers at New York’s ground zero after the terrorist attacks of September 11, 2001.

As the chairwoman of a Senate subcommittee investigating complaints that workers were misled about air quality after the collapse of the twin towers, Senator Hillary Clinton, the Democrat front-runner for the presidential nomination, confirmed last week that she is considering calling Rudolph Giuliani, the former mayor of New York and the leading Republican contender, to testify at a public hearing.

The health committee’s inquiry has presented Clinton with an intriguing political choice with potentially volatile repercussions for the presidential race. By calling Giuliani as a witness, she could place him in the awkward position of having to submit to her senatorial authority and face a grilling that might dent the heroic image he acquired for his response to the 9/11 attacks.

At the same time Democratic aides have acknowledged that Clinton might risk being accused of playing politics with a tragedy. Giuliani said last week that he was happy to testify “to anybody who has a fair mind about it” and is “approaching it from a nonpolitical point of view”.

Clinton said last week she was still “looking at all the people available to testify”. But other prominent New York Democrats have demanded Giuliani should answer questions about whether he could have done more as mayor to protect the workers who spent weeks digging in dust and dirt that is now considered to have been dangerously toxic.

“Who made decisions, if any, that resulted unnecessarily in a lot of people getting sick?” asked Congressman Jerrold Nadler, whose district includes the World Trade Center site.

The Senate inquiry follows claims by at least 9,000 New Yorkers that they are suffering from lung and stomach ailments they blame on the toxic rubble at ground zero. Their cause attracted national attention in January when Clinton invited the son of Cesar Borja, a stricken New York policeman, to attend President George W Bush’s state of the union address as her guest. Borja was waiting for a lung transplant but died of respiratory failure a few hours before Bush spoke.

Despite subsequent revelations that Borja may not have spent long periods working at ground zero, the health inquiry has become a potential embarrassment for Giuliani, who is increasingly being forced to defend the actions that made him a national icon. He was dubbed “America’s mayor” by Oprah Winfrey, the talk show host.

Last month Giuliani was fiercely criticised by a firemen’s union that has never forgiven him for halting the search for bodies. The mayor was anxious at the time that clean-up operations should begin, but more than five years later body parts are still being found around ground zero.

He has also been criticised for failing to provide the New York Fire Department with more modern communications equipment, which was requested after the attack on the World Trade Center in 1993.

Sally Regenhard, whose son was one of the 343 firefighters killed in the attacks, said last week she had no problem if Giuliani ran for president on his record of reducing New York crime, “but when he runs on 9/11, I want the American people to know he was part of the problem”.

Clinton has been swift to exploit her rival’s difficulties. She earned three standing ovations at a firemen’s convention in Washington this month and promised that, as president, she would “take care of the people who have taken care of us”.

Yet she has also been careful to avoid direct criticism of Giuliani, and several analysts have warned that a face-to-face challenge on his 9/11 record would be risky. “It’s not smart strategy. It’s too early,” Douglas Muzzio, a political science professor at Baruch College, told the New York Sun. Instead she should “take the high road”.

Behind the fencing over health issues lies mounting pressure on both Clinton and Giuliani camps to maintain the healthy opinion poll leads that each has staked out in the race for their respective party nominations. In a Gallup poll published last week by USA Today, Giuliani’s lead over Senator John McCain of Arizona had shrunk from 24 points at the beginning of March to nine points three weeks later.

The former mayor also faces a possible challenge from former senator Fred Thompson, a Hollywood actor turned conservative politician who shot from nil to 12 points in the same poll, simply by announcing that he was considering entering the race.

In the Gallup poll, Clinton’s lead over Senator Barack Obama, who is seeking to become the first African-American president, narrowed slightly from 14 points to 13 points.

On Thursday a new Time magazine poll gave Clinton only an eight point lead over Obama, with former senator John Edwards apparently benefiting from a major sympathy boost after the announcement that his wife’s cancer has become incurable. Edwards jumped nine points to 26 points, only four behind Obama.

If Clinton decides to call Giuliani as a witness, her health inquiry may turn into an intriguing preview of next year’s presidential debates. But the ex-mayor warned the senator last week that he expected any testimony about health issues at ground zero to be “above politics”, and he also reminded her he was a possible victim himself.

“I was probably there as often as anybody, as you know,” he said. “So any exposure to anything bad that anyone else has, I personally had.”

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