Much of what she is likely to say is motivating her is well known.

While her husband promised in 1991 to restore the American dream to “the forgotten middle class,” Mrs. Clinton is expected to offer herself as best equipped to reverse the sense among many voters that the middle class is already out of reach.

Indeed, her campaign will shy away from the characterization “middle class” — because, her advisers say, the term no longer connotes a stable life — and instead use the term “everyday Americans.”

For months, Mrs. Clinton has lamented the stagnant wages holding back lower-income people and the concentration of wealth among a sliver of the wealthiest, a sentiment echoed in her first public remarks as a 2016 candidate.

In the video, she allows a series of people — nonactors, her campaign said — to stand for those ideas: A black couple looks ahead to the birth of a child, Latino brothers beam with excitement about starting a business, two men hold hands anticipating their wedding, a young Asian-American woman looks ahead to her first job and a white woman who says she will retire soon talks about “reinventing” herself.

“I’m getting ready to do something, too,” Mrs. Clinton says. “I’m running for president.”

Every White House aspirant eventually has to face the question that famously reduced Edward M. Kennedy to incoherence when the CBS News correspondent Roger Mudd asked it just before his run for president in the 1980 election: “Why do you want to be president?” An honest reply, no matter the candidate, would most likely entail healthy doses of ambition and ego, along with a concern for the fate of the nation or for the greater good.