“They need to wake up and pay attention to what people actually want,” Ms. Conner said of Democratic leaders. “There are so many progressive policies that have widespread support that mainstream Democrats are not picking up on, or putting that stuff down and saying, ‘That wouldn’t really work.’”

Voters like Ms. Conner may not represent a controlling faction in the Democratic Party, at least not yet. But they are increasingly rattling primary elections around the country, and they promise to grow as a disruptive force in national elections as younger voters reject the traditional boundary lines of Democratic politics.

Energized to take on President Trump, these voters are also seeking to remake their own party as a ferocious — and ferociously liberal — opposition force. And many appear as focused on forcing progressive policies into the midterm debate as they are on defeating Republicans.