A long article on the Logan Act mentions this failed attempt to repeal it, during the Carter administration:
After decades of deflecting Logan Act entreaties, an exasperated Department of Justice began pushing for its repeal. Congressman Paul Simon wrote to the Attorney General in 1978 to ask whether he wished to see the Logan Act excised in a pending recodification of federal criminal statutes. A DOJ spokesman responded unequivocally that “the Department does support the repeal of the Logan Act.” It had in fact “supported” a recent repeal bill, “stated its opposition” to an amendment reinstating the Act, and in public testimony “again stated our belief that the Act should be repealed.” Importantly, the Department’s position was “based on policy considerations,” not constitutional misgivings. DOJ was perfectly willing to set aside these concerns by “some showing of need for the Act.” But the Department could detect “no irreparable injury to the United States because of a failure to prosecute” Logan Act violators. [...]
DOJ evidently reached out to lobby key senators as well. Denouncing the Logan Act as a useless archaism, Senator Edward Kennedy disclosed that repeal had been “urged upon us by the Justice Department. . . . [T]hey urged us to strike it.” Characterizing the Act as “one of the most antiquated provisions in the current code,” the Department had “asked Senator McClellan, Senator Hruska, and the rest of us to take it out.”
The repeal effort failed, due to the tenacity of a single U.S. senator. And so to the present day, administrations stammer out half-responses to unwanted Logan Act inquiries.
Alas, the senator who is supposedly responsible for this feat (of defeating the repeal) is not named therein. So, who was that senator? (I'm guessing this involved a threat of filibuster.)