ST. LOUIS — An energized Hillary Clinton took aim at Donald J. Trump and Bernie Sanders on Saturday. But at least one shot backfired.

Mrs. Clinton accused Mr. Sanders of distorting her record and said the Vermont senator, who has made a single-payer health care system a signature part of his campaign, had not always been such an advocate on the issue.

She said she has “a little chuckle to myself” when she thinks about the current debates over health care. “I don’t know,” Mrs. Clinton said. “Where was he when I was trying to get health care in ’93 and ’94?”

The answer: “Literally, standing right behind her,” a Sanders spokesman, Mike Casca, said on Twitter, posting a photo from a 1994 news conference that shows Mr. Sanders next to Mrs. Clinton when the then first lady spoke about the White House’s proposed health care overhaul.

A spokeswoman for Mrs. Clinton, Jennifer Palmieri, had a comeback.

“Exactly, he was standing behind her,” Ms. Palmieri said. “She was out in front.”