WASHINGTON — After spending a week laying the groundwork for a 2020 message that seems loosely defined as us versus them, President Trump on Tuesday repackaged campaign rally red meat into child-size portions for a gathering of young supporters.

At the Teen Student Action Summit 2019, Mr. Trump again attacked one of the four Democratic congresswomen of color he has assailed over the past week and falsely claimed that undocumented immigrants were voting en masse.

“She’s vicious,” Mr. Trump said at the gathering, a conservative convention in Washington, about the congresswoman, Representative Rashida Tlaib of Michigan. “She’s like a crazed lunatic.”

“Who elected her?” he added. “She’s screaming like a total lunatic at one of our rallies.”

The president’s nearly 80-minute speech was meant to inspire the 1,400 or so high school and early college-age supporters in attendance, most of whom are part of the most diverse generation in history. The group attending the convention was largely white.