With this month’s revelations that Hunter Biden directly contacted American officials for the benefit of foreign clients in Ukraine and allegedly Romania, and with Mr. Biden facing a new trial next month stemming from charges of tax evasion for the millions he received from foreign sources, the time has come to finally charge him as an unregistered foreign agent.

Originally enacted in 1938, the primary regulations targeting foreign lobbyists, known as the Foreign Agents Registration Act (FARA), are straightforward, requiring Americans who are paid to push foreign interests to register their work with the federal government. It doesn’t make the practice illegal — lobbying is, thanks to the First Amendment, still a constitutionally protected right — but it does bring transparency to an otherwise opaque world, forcing foreign lobbyists to disclose what they’re doing, who they’re doing it for, and how much they’re being paid in the process. While FARA was unfortunately largely unenforced for decades after it took effect, the law has seen new life in recent years as prosecutors have finally begun going after unregistered foreign agents.

We’ve known that myriad foreign companies and foreign oligarchs have targeted Mr. Biden, tossing significant sums at him while his father served as vice president. With each revelation, and with each new foreign client revealed, the president’s detractors have wailed that the younger Mr. Biden violated foreign lobbying laws, which required him to disclose what he was doing abroad — as well as reveal the Americans he’d been targeting on behalf of his foreign benefactors.

Mr. Biden’s highly questionable foreign dealings have for years appeared more smoke than fire; there was previously no evidence that he illicitly lobbied any American officials. Compared to figures such as the former Trump campaign chief Paul Manafort, who was caught out as an illegal foreign lobbyist (among plenty of other crimes), Mr. Biden’s alleged foreign lobbying misdeeds appeared to fall short of crossing the line into criminality. They may have put a lie to President Biden’s claimed concerns about foreign influence campaigns, but they were never worthy of formal charges. With the new details, though, that has changed, giving prosecutors the opening to pursue the president’s son as one of the most prominent foreign lobbyists the United States has ever seen.

Take the new reporting on Mr. Biden’s work for the Ukrainian gas firm Burisma. As The Times reported, Mr. Biden, who served as a board member on the firm, “sought assistance” from American officials “for a potentially lucrative energy project,” writing at least one letter to the U.S. ambassador to Italy in 2016 to help aid Burisma — actions that, in other words, appear to be direct lobbying of an American diplomat for the benefit of his foreign firm.

Meanwhile, prosecutors claimed last week that a local Romanian magnate accused of corruption hired Mr. Biden not for any legal expertise, but to convince American officials to work with Romanian authorities to help thwart a criminal investigation into the magnate’s finances. It was, according to a potential witness cited by the government, an “attempt to influence U.S. government agencies” — part of a broader pattern of Mr. Biden “perform[ing] almost no work in exchange for the millions of dollars he received” from assorted foreign entities. Moreover, prosecutors claim that the Romanian deal was specifically structured to dodge basic foreign lobbying transparency — drafted as a “property management” arrangement, in which Mr. Biden received at least $1 million — all to avoid “political ramifications” for President Biden.