If there was any question about how Senator Bernie Sanders would go after Hillary Rodham Clinton in the first Democratic presidential primary debate on Tuesday evening, it was answered on Saturday in the form of a press release attacking his rival’s vote authorizing the Iraq War.

With a headline “Sanders’ Foreign Policy Experience,” the press release focuses entirely on Mr. Sanders’ opposition to the Iraq war when he was a congressman from Vermont.

Mrs. Clinton’s vote in 2002 authorizing the use of force of Iraq, which then-President George W. Bush sought, became a defining issue in her 2008 presidential primary against then-Senator Barack Obama. Mr. Obama used that vote to raise questions about Mrs. Clinton’s judgment and her character.

Mrs. Clinton never explicitly called that vote a mistake during that primary, despite pleas from some of her advisers to do just that. Instead, she moved away from it slowly, ultimately saying that had she known then what she later knew, she wouldn’t have voted that way.

She finally said she was “wrong” in her memoir, “Hard Choices,” which was released in 2014. But this debate will be the first event with her fellow candidates where she will make that point.

The Sanders campaign indicated it would make a contrast with Mrs. Clinton by highlighting a speech Mr. Sanders made on the House floor in October 2002, opposing the American invasion, which it called “one of the worst foreign policy blunders in the history of the United States.”

The release pointed out that in his speech, Mr. Sanders cited massive American military casualties and Iraqi civilians, the “effectiveness of the war” on “international terrorism,” the potential costs, the precedent that it would set and “who would govern Iraq after Saddam Hussein was overthrown and what role the United States would play in the civil war that followed.”

Mrs. Clinton’s Democratic opponents in recent weeks have criticized her for policy position changes, including on the Trans-Pacific Partnership trade deal, which began being worked on while she was still the Secretary of State in Mr. Obama’s first term.