She went into Super Tuesday battling a wave of momentum for Obama, who had surged in national polls on his message of change.
"I look forward to continuing our campaign and our debate about how to leave this country better off for the next generation," Clinton told supporters late on Tuesday.
Because Democrats distribute delegates in proportion to their vote statewide and in individual congressional districts, candidates can come away with large numbers of delegates even in states they lose.
Obama maintained his strong showing among black voters but also expanded support among whites, winning 40 percent in Georgia, exit polls said. Clinton won heavy support from women and Hispanics, exit polls showed.
McCain won in Arizona, California, Connecticut, Delaware, Illinois, Missouri, New Jersey, New York and Oklahoma.
Huckabee, a Baptist preacher and former Arkansas governor, won in Alabama, Arkansas, Georgia, Tennessee and West Virginia.
Romney won in Alaska, Colorado, Minnesota, Montana, North Dakota and Massachusetts, where he served as governor, and Utah, which has a heavy concentration of Mormons. Romney would
be the first Mormon president.
Huckabee's wins were fueled by strong support from evangelical Christians, and he split votes with Romney among conservatives unhappy with McCain.