OKLAHOMA CITY — At the age of 16, an “Okie” known to her friends as Liz Herring and to her family as “Betsy” graduated from Northwest Classen High School, her professional prospects limited by her gender and her politics fairly conservative.

On Sunday afternoon, she returned to the high school more than a half-century later as Senator Elizabeth Warren of Massachusetts, a left-wing candidate in the Democratic primary who has set out to shatter one of the highest glass ceilings imaginable: becoming the first woman to be elected president.

“I spent a lot of hours in this gymnasium,” Ms. Warren said. “I never thought I’d be down here, on the floor, doing something like this. But you know what — you don’t get what you don’t fight for.”

It was a personal and political homecoming at a key moment in Ms. Warren’s candidacy. Her monthslong rise in national polling has stalled in recent weeks. In response, Ms. Warren and her campaign team have made some marginal changes: a shorter stump speech in favor of more audience questions, and a greater willingness to have Ms. Warren criticize her Democratic rivals, particularly more centrist opponents such as former Mayor Michael R. Bloomberg of New York, Mayor Pete Buttigieg of South Bend, Ind., and the race’s front-runner, former Vice President Joseph R. Biden Jr.