Prominent US lawmaker Lindsey Graham, who chairs the Senate Judiciary Committee set to consider President Donald Trump's Supreme Court nominee next week, refused on Friday to be tested for the coronavirus.

The South Carolina ally of Trump said no to a testing request by his Democratic rival in their Senate race, ahead of their scheduled debate in the southern state.

Graham's move prompted the debate with Jaime Harrison to be scrapped, with separate interviews of the candidates conducted instead.

But the issue could prove to be a critical one in Washington.

Graham is expected to preside in person over next week's hearings on the confirmation of conservative Judge Amy Coney Barrett to fill the Supreme Court seat made vacant by the death last month of liberal justice Ruth Bader Ginsburg.

Should Graham test positive and be forced into 10 days or more of quarantine, that could postpone the hearings, something Republicans have fiercely opposed.

Two other Republican senators on the committee, Mike Lee and Thom Tillis, tested positive after attending a September 26 ceremony where Trump announced Barrett as his nominee. 

Most attendees were not wearing masks and several have since tested positive.

Of the Judiciary Committee's 22 members, 10 are age 68 or older. They include the Senate's two oldest senators, Democrat Dianne Feinstein and Republican Chuck Grassley, both 87.

Feinstein and other Democrats on the panel penned a letter to Graham, 65, saying he is putting Barrett and the senators at "serious risks" by holding the hearing, especially without mandatory testing for participants.

"We urge you against unsafely moving forward with these hearings while no clear testing regime is in place to ensure that they do not become another super-spreader of this deadly virus," wrote the Democrats.

One of the signatories was Senator Kamala Harris, who is Democratic nominee Joe Biden's vice presidential running mate.

In defending his own refusal to be tested, Graham tweeted that his challenger Harrison was "putting himself above others" by insisting on different treatment for him and Graham than for other South Carolinians.