Obama was in Chicago, where he told a noisy election night rally, "Our time has come. Our movement is real. And change is coming to America."
Polling place interviews with voters suggested subtle shifts in the political landscape.
For the first time this year, McCain ran first in a few states among self-identified Republicans. As usual, he was running strongly among independents. Romney was getting the votes of about four in 10 people who described themselves as conservative. McCain was wining about one-third of that group, and Huckabee about one in five.
Overall, Clinton was winning only a slight edge among women and white voters, groups that she had won handily in earlier contests, according to preliminary results from interviews with voters in 16 states leaving polling places.
Obama was collecting the overwhelming majority of votes cast by blacks -- a factor in victories in Alabama and Georgia.
Clinton's continued strong appeal among Hispanics -- she was winning nearly six in 10 of their votes -- was a big factor in her California triumph, and in her victory in Arizona, too.
McCain, the early Republican front-runner whose campaign nearly unraveled six months ago, won in New York, New Jersey, Connecticut, Missouri, Delaware and his home state of Arizona -- each of them winner-take-all primaries. He also pocketed victories in Oklahoma and Illinois.
Huckabee, the former governor of Arkansas, won a series of Bible Belt victories, in Alabama, Georgia and Tennessee as well as his own home state. He also triumphed at the Republican West Virginia convention, and told The Associated Press in an interview he would campaign on. "The one way you can't win a race is to quit it, and until somebody beats me, I'm going to answer the bell for every round of this fight," he said.