CINCINNATI — They stormed the stage together wearing similarly colored clothes — hues that almost perfectly matched the bold blue of Hillary Clinton’s presidential campaign logo. With a “Stronger Together” sign hanging in the background and a Katy Perry pop song blaring from the speakers, they cheered each other on like old pals, cracking jokes about Donald J. Trump and pointing with enthusiasm at a young supporter who waved a placard that read “Girl Power.”

Senator Elizabeth Warren of Massachusetts, a towering political figure among today’s liberal Democrats, brought her energy, folksy appeal and populist roar to a candidate not known for energizing crowds.

For Ms. Warren, the joint event with Mrs. Clinton here on Monday, the first time the two Democrats campaigned onstage together, was a moment for her to elevate her profile as the liberal voice of the party and a favorite to be vice president.

For Mrs. Clinton, it was a chance to woo the party’s liberal wing and convince economically hard-hit voters that she, too, is a populist champion running for president to improve their lives.