He has undermined efforts by career diplomats to deliver messages to Washington about corruption and democratic backsliding in Hungary. And he has privately acted as a broker for Mr. Orban’s point of view, taking positions contrary to United States policy, according to interviews with roughly two dozen current and former American and foreign officials as well as others who have worked with Mr. Cornstein.

Secretary of State Mike Pompeo weighed in on British politics during a closed-door meeting with Jewish leaders, saying he would not wait for Labour Party leader Jeremy Corbyn to become prime minister of Britain to “push back” against him or any future actions he might take against Britain’s Jews.

At first, it looked to be more of the same: another week, another rash of false accusations of antisemitism launched by Republicans against one of the two new Muslim-American congresswomen. This time it was Rashida Tlaib who found herself under attack for comments she made on a Yahoo News podcast.

Democrats hoped to put their wrenching intraparty debate over anti-Semitism to rest when they passed a catchall antibigotry resolution in the House this month, but Senate Republicans, eager to court American Jews outraged by the rise of anti-Semitism, have other plans.