Hillary Clinton ended years of speculation about her plans Sunday and launched a second campaign for president, asking voters to keep the same party in control of the White House for 12 straight years — a request that has been granted only rarely in modern history. “Everyday Americans need a champion, and I want to be that champion,” the 67-year-old Clinton says in a video her campaign released Sunday. In it, she evokes a populist spirit: “The deck is still stacked in favor of those at the top.” The former secretary of state — the runaway favorite for the Democratic nomination — signaled what her advisers have been saying for months: that she is taking nothing for granted. “I’m hitting the road to earn your vote,” she says in the video, which features a diverse collection of Americans — people of many races, straight and gay couples, a Spanish speaker, senior citizens and young couples — all discussing their challenges and opportunities. Clinton, the first major Democrat to formally declare her candidacy, was also the front-runner for the party’s nomination eight years ago. This time she faces a smoother path, with no prospective rival looking as strong as the one who beat her in 2008, then-Sen. Barack Obama. If she wins the nomination, Clinton will need to make the case that the White House should remain in Democratic hands after Obama’s two terms. History suggests that is a tough sell.Since 1953, when Harry Truman left office, voters have kept the same party in the White House for three terms only once, in 1988, when Republican George H.W. Bush followed President Ronald Reagan. Bush’s path was eased by Reagan, whose approval rating was 57% in the month ahead of that election, according to Gallup. An expanded version of this report appears on WSJ.com.