Representative Alexandria Ocasio-Cortez of New York, one of the most prominent progressives in the Democratic caucus, said late Monday that she stood behind President Biden.

“I have spoken to the president over the weekend,” Ms. Ocasio-Cortez told reporters outside the Capitol. “I have spoken with him extensively. He made clear then and he has made clear since that he is in this race. The matter is closed.”

She highlighted his efforts on Monday to reiterate that message. “Joe Biden is our nominee,” she said. “He is not leaving this race, he is in this race, and I support him.”

Ms. Ocasio-Cortez said that it was essential for Democrats to turn their focus back to former President Donald J. Trump and what he would do if elected again. She told Mr. Biden to “increasingly commit to the issues that are critically important to working people across this country” — like expanding Medicare and Social Security and making housing more affordable, she said.

“What I think is critically important right now is that we focus on what it takes to win in November,” she said. “Because he is running against Donald Trump, who is a man with 34 felony convictions — that has committed 34 felony crimes. And not a single Republican has asked for Donald Trump to not be the nominee.”

Members of Congress returned to Washington on Monday after a recess, and Democrats have been meeting and discussing their path forward.

While a handful of House Democrats have called for Mr. Biden to step aside since his debate performance last month, he has received politically important support from others, including members of the Congressional Black Caucus and the Congressional Hispanic Caucus — and now at least one member of the so-called Squad, a group of progressives of which Ms. Ocasio-Cortez is a part.