[What you need to know to start the day: Get New York Today in your inbox.]

WASHINGTON — Not so long ago, left-wing activists were dismissed as fringe or even kooky when they pressed for proposals to tax the superrich at 70 percent, to produce all of America’s power through renewable resources or to abolish Immigration and Customs Enforcement.

Then along came Alexandria Ocasio-Cortez — and her social-media megaphone.

In the two months since her election, Ms. Ocasio-Cortez has had the uncanny ability for a first-term member of Congress to push the debate inside the Democratic Party sharply to the left, forcing party leaders and 2020 presidential candidates to grapple with issues that some might otherwise prefer to avoid.

The potential Democratic field in 2020 is already being quizzed about her (Senator Kamala Harris praised her on “The View”), emulating her digital tactics (Senator Elizabeth Warren held an Instagram chat in her kitchen that looked much like one of Ms. Ocasio-Cortez’s sessions) and embracing some of her causes.

Ms. Warren and Senator Cory Booker, among others, have recently endorsed the idea of a “Green New Deal,” a call to reimagine an environment-first economy that would phase out fossil fuels. Ms. Ocasio-Cortez thrust that issue into the national dialogue after she joined a sit-in protest in the office of then-incoming House speaker, Representative Nancy Pelosi, in one of her first, rebellious acts in Washington.