The only clue that this was an unusual event was sartorial: Several women wore headscarves, and a man sported a stylish kaffiyeh. The phone bank is one of dozens of Muslims for Hillary events that the Clinton campaign has arranged this year, part of what the campaign contends is an unprecedented effort to court a small but growing population.

On Friday, the Clinton campaign released an ad featuring Khizr Khan, the father of U.S. soldier Humayun Khan, who was slain fighting in Iraq. The ad, set to air in seven battleground states, is notable for its direct invocation of Islam. Telling the story of his son’s death saving comrades, Khan says, “He was 27 years old, and he was a Muslim American. I want to ask Mr. Trump, would my son have a place in your America?”

There are an estimated 3.3 million Muslims in the United States—a tiny portion of the overall population, especially excluding non-citizens and people too young to vote. But their numbers are expected to grow quickly, and moreover, American Muslims make up a noticeable chunk of the electorate in several swing states, including Michigan, Virginia, Florida, and North Carolina. (The Muslim vote in Florida for George W. Bush, which was greater than his margin of victory, is sometimes credited with making Bush president in 2000.)