They entered Tuesday's contests closely divided in the hunt for the 2,025 delegates needed for the nomination: 1,281 for Obama and 1,218 for Clinton.
The day's biggest prize was Wisconsin, where 74 delegates were up for grabs and polls showed the two in a statistical dead heat. Neither candidate made the long trip to campaign in Hawaii, where 20 delegates were to be decided by a caucus.
Obama, born in Hawaii and living in Wisconsin's southern neighbor, hoped to build on his string of eight straight wins. Clinton's campaign played down her chances in Wisconsin, but was hoping to beat expectations to give her struggling candidacy new life.