Running as a progressive and taking left-wing stances on issues such as health care (such as supporting "Medicare for All"), Rabb notched the endorsement of left-leaning organizations as well as nationally known figures such as Rep. Alexandria Ocasio-Cortez, who campaigned with him last week.
As results came in Tuesday night, supporters rushed to the stage as Rabb took the podium, joined by his sons and parents.
Rabb told the crowd that many doubted his chances in the race - and said he even considered dropping out just a few months ago.
But he said ultimately his sons were his inspiration for running.
He framed the victory as a collective effort.
"This is what our district looks like, and we deserve the very best. They're going to try to tear us apart. We're not going to let that happen because we see our power when we believe in collective action. We are indomitable," said Rabb to supporters.
Why this race matters
Rabb is likely to succeed U.S. Rep. Dwight Evans, age 71, who has served in the House of Representatives since 2017 and announced last June that he is retiring from Congress at the end of his term.
While this race is not one of the "battlegrounds" for the U.S. House, it does represent a distinct example of generational change among Democrats, given that it shows a longtime representative ceding his seat to a new generation of lawmakers.
Rabb defeated a crowded primary field that included Sharif Street, Dr. Ala Stanford and Shaun Griffith.