Experts said that conflict in the Middle East has often sparked violence in the United States, but they are worried that the recent attacks were evidence of a worrisome trend that has been accelerated by the fighting in Israel and Gaza. “This does feel quite different,” Jonathan Greenblatt, chief executive of the Anti-Defamation League, said in an interview Sunday. In 2019, he said, the group identified more than 2,100 antisemitic incidents, including assault, violence and harassment, which was more than in any year since the group began tracking such episodes in 1979. And 2020, a year when many Americans stayed home because of the coronavirus pandemic, still saw the third-highest number on record, Greenblatt noted. Congressional lawmakers condemned the recent violence, saying it was deeply troubling.