WASHINGTON — Eight years after Hillary Rodham Clinton was haunted by her Senate vote to authorize the use of force in Iraq, another leading White House hopeful is struggling with questions about the war.

And just as Mrs. Clinton seemed torn then between mollifying Democratic primary voters who were against the war and positioning herself to run in the general election, Jeb Bush, a Republican, appears deeply conflicted. He is grappling with how to remain loyal to his older brother, George W. Bush, while acknowledging what has become a mainstream view in both parties: that there would not have been an invasion if policy makers had known in 2003 that Iraq did not have weapons of mass destruction.

In Arizona on Thursday, with his fourth different answer of the week, Mr. Bush, the former Florida governor, finally conceded that, if he had had such knowledge, he “would not have gone into Iraq.”

For Mrs. Clinton in 2008, mishandling what her opponent Barack Obama called “the biggest foreign policy disaster of a generation” created an opening for him and ultimately helped undermine her campaign.