The prolonged prologue to Hillary Rodham Clinton’s second run for the White House will reach its suspenseless conclusion on Sunday: The former secretary of state, senator and first lady is to announce that she will indeed seek the Democratic nomination for president.

Mrs. Clinton is expected to begin her campaign with a video message on social media, followed by a visit to important early-primary states next week, said two people briefed on her plans.

But for all the attention paid to how Mrs. Clinton would reveal her 2016 candidacy, little has been said about her reasons for mounting another presidential bid. Her campaign rollout is expected to provide voters, particularly users of Twitter, Facebook and other social media, a succinct rationale that she is best positioned to address an American electorate that has seen virtually stagnant wages for middle-income earners over the last 15 years.