WASHINGTON — Freshman orientation for new members of Congress had barely begun when Alexandria Ocasio-Cortez, the 29-year-old darling of the progressive left and newly elected congresswoman from New York, visited Nancy Pelosi’s office for the first time. She was not there to meet the House Democratic leader.

She was there to protest.

“This is not about personality, this is not about rebuke, this is not about confrontation — it’s about making sure that we are getting the job done,” she declared, moments before she spoke to young activists who staged a sit-in to demand a Green New Deal to address climate change. Of Ms. Pelosi, she said, “I think she really appreciates civic engagement.”

Ms. Ocasio-Cortez’s noisy Washington debut, and her uncanny knack for grabbing the spotlight, underscores the difficulties that await Democratic leaders as they try to unify an unusually diverse incoming class. Newcomers in both parties descended on the capital Tuesday for the start of a biennial Washington rite — freshman orientation — that seems unusually freighted with seriousness in the age of President Trump.

Going to Congress is typically like going to college. Newcomers make new friends, look for housing, pick up new cellphones (this year, they are also being issued iPads) and take pictures for their temporary ID badges, to be replaced by House pins once they are sworn in. They take nonpartisan seminars about the nuts and bolts of joining Congress — how to hire staff and set up an office. And they go to parties, lots of parties, which members of Congress prefer to call “receptions.”