Billing records show that former Senator Fred Thompson spent nearly 20 hours working as a lobbyist on behalf of a group seeking to ease restrictive federal rules on abortion counseling in the 1990s, even though he recently said he did not recall doing any work for the organization.

According to records from Arent Fox, the law firm based in Washington where Mr. Thompson worked part-time from 1991 to 1994, he charged the organization, the National Family Planning and Reproductive Health Association, about $5,000 for work he did in 1991 and 1992. The records show that Mr. Thompson, a probable Republican candidate for president in 2008, spent much of that time in telephone conferences with the president of the group, and on three occasions he reported lobbying administration officials on its behalf.

Mr. Thompson’s work for the family planning agency has become an issue because he is positioning himself as a faithful conservative who is opposed to abortion.

Earlier this month, Mr. Thompson disputed accounts by the group’s former president and others, saying through a spokesman that he had “no recollection” of doing anything to aid the group’s efforts to overturn a rule banning federally financed clinics from dispensing information about abortion to pregnant women. At most, said Mr. Thompson’s spokesman, Mark Corallo, he “may have been consulted by one of the firm’s partners who represented this group.”