The A.D.L. is concerned about an increase in reported hate crimes and online harassment since the start of the presidential campaign. In New Mexico, the group has been working with Muslim and immigrants’ rights groups to respond to and prepare for incidents of hate. The A.D.L. also offers training and online resources to help Jewish communities recognize suspicious activity and keep facilities safe.

“One of the goals of cyberharassment and threats of violence is to disrupt a community and cause fear,” says Ms. Halevi. “When the community is prepared, it helps to stop that from happening.”

For Mr. Yellin, one way to fight hate is to talk about it. A few weeks after he got the threatening messages, he wrote about them on his blog: “This blatant, open anti-Semitism must not be allowed to become the new normal.”

He encourages others who have been threatened to make the incidents public if they feel safe doing so. “Nothing is going to change if people don’t know,” he says.

Here are some reports of hate crimes and harassment that have drawn public attention in recent days.

• On Jan. 30, members of a family in Orlando, Fla., discovered racist notes including swastikas on the windshields of three of their cars. One tire on each car was slashed. Family members believe they may have been targeted because one of them is an organizer for Black Lives Matter.

• Anti-Muslim graffiti was discovered at a mosque in Roseville, Calif., on Jan. 31. Earlier in the month, a mosque in nearby Davis, Calif., was also vandalized.