Democrats in Wisconsin and Hawaii make their choices later today as Hillary Clinton seeks to end Barack Obama's winning streak ahead of big contests in March.
Public opinion polls show Mr Obama and Ms Clinton in a tight race in Wisconsin, where Mr Obama aims to extend his string of eight straight victories in Democratic nominating contests. Mr Obama, a Hawaii native, is a heavy favourite in that state.
Up for grabs in the two states are 94 delegates to the August convention that selects the Democratic presidential nominee in November's election.
The pair traded a sharp exchange yesterday over Mr Obama's uncredited use of a political ally's speech lines.
Mr Obama has a slight lead in pledged delegates won in state presidential contests. The pair already have turned their attention to March 4th contests in two of the biggest states, Ohio and Texas, which have 334 convention delegates at stake.
Ms Clinton is the early favourite in both, although one public opinion poll in Texas yesterday showed the race in a statistical dead heat. Ms Clinton headed to Ohio last night. and Mr Obama travels to Texas today.
Voting ends in Wisconsin at 2am tomorrow with results expected soon afterward.