Mrs. Obama and Mrs. Clinton were once adversaries — Mrs. Obama took umbrage in private during the 2008 Democratic primary race at Mrs. Clinton’s criticisms of her husband — but their goals are now inextricably aligned.

Mrs. Obama “is a very trusted voice who has not seemed to have an agenda, and someone whose husband ran against Hillary and is now a big believer in her, so those words coming from Michelle Obama are especially meaningful,” said Jennifer Palmieri, the communications director for Mrs. Clinton’s campaign.

The first lady has also demonstrated a knack for using pop culture and social media to appeal to the core coalition that propelled her husband to the White House — young voters, minorities and single women. Those elements will also be part of her push for Mrs. Clinton, aides said, though they declined to detail her itinerary because it is still developing.

“She’s able to speak in an honest and relatable way that the American people can connect with,” said Jennifer Psaki, Mr. Obama’s communications director and a strategist on his presidential campaigns.

That is the part of her job that Mrs. Obama has always said she relishes the most. Unlike Mrs. Clinton, a politics junkie and policy wonk who as first lady wanted an office in the West Wing and led Bill Clinton’s health care task force, Mrs. Obama has avoided hard-edged debates. She has instead prioritized more universally popular causes such as her “Let’s Move” campaign for healthy eating and exercise, the Joining Forces initiative to support veterans and their families, and her “Let Girls Learn” effort to expand educational opportunity for women and girls.

“When Barack was talking about running, I was like, ‘Are you crazy?’ — I mean, ‘would you just, like chill out and do something else with your life,’” Mrs. Obama told Oprah Winfrey in June during an appearance in Washington. During their discussion, the first lady said she had determined early on to "protect my time” jealously, and was looking forward to the day when she could finally leave her home without consulting the Secret Service.