Many Muslim activists and operatives already felt all but neglected by the Democratic presidential candidates. Now, as most 2020 contenders are set to skip what is billed as the largest annual gathering of Muslims in the country this weekend, they’re growing increasingly fearful that Democrats may miss an opportunity to mobilize their community against President Donald Trump next year. Only two Democratic hopefuls, Bernie Sanders and Julián Castro, have confirmed they are going to the Islamic Society of North America’s convention. Organizers said they invited every candidate polling above 1 percent to participate in the presidential forum, as well as...